Invoke Duplicity and the SCAGtrips

[BarneyBent]

But being able to cast Booming Blade does not guarantee you the ability to make the attack. We know this, because the spell description explicitly tells us that if you can't make the attack, the spell fails.

Doesn’t need to guarantee you, because you can make the attack. The attack is a part of casting the spell (which is why the spell fails if you can’t do it). Therefore, when you are using the duplicate’s space to cast the spell, that includes making the attack.

That’s why it’s different to something like Spell Sniper or Distant Spell. They modify the range of the spell, not your position for the purposes of casting it (and therefore making the attack).

[BarneyBent]

This really isnt hard, people are trying to twist the words to make it read a certain way.

Just because the Melee attack is part of the ACTION used to cast the spell, doesn't mean the melee and the spell are one and the same.

Example: My hand is part of ME. My face is part of ME. My hand is NOT part of my face. Just because i can reach the roof with my hand, does NOT mean i can reach the roof with my face. The fact that they're both part of ME doesn't mean they occupy or effect the same space.



Similarly, just because you may cast the spell as part of your action in the illusions space, it doesn't mean the melee weapon attack is triggered from that same space. They're both part of the same action, but that doesn't entail the melee attack triggering where the spell is cast.

The spell is your "hand", the melee attack is your "face" the action is "you". They're both part of you, but just because your hand is able to reach the enemy via this exception, does not entail that your face can reach the enemy.

Nope! That’s not how actions work. It’s tempting to think that way, but the action is casting the spell.

Want evidence? The casting time is “1 action”. To use your analogy, the hand is you, you are the hand.

When you cast a spell, you take the Cast A Spell action. That is your action. You get no others unless you Action Surge it have Haste or something similar. When you make the melee attack, it is part of that Cast A Spell action that is also the full casting time of the spell in question. An action isn’t a pocket of time you get to spend doing things in combat. It is a set of things you can choose one of which to do. Casting a Spell is one of those. Using that action to also make a weapon attack simply means that the weapon attack is part of the spell’s casting.

You are the one twisting RAW because of a misunderstanding of how actions work and what they mean. And it’s a fair misunderstanding, because it’s counterintuitive, especially when we’re used to thinking in terms of spending resources. But your action isn’t something you spend, it’s something you choose to do.

[Tallytrev813]

Nope! That’s not how actions work. It’s tempting to think that way, but the action is casting the spell.

Want evidence? The casting time is “1 action”. To use your analogy, the hand is you, you are the hand.

When you cast a spell, you take the Cast A Spell action. That is your action. You get no others unless you Action Surge it have Haste or something similar. When you make the melee attack, it is part of that Cast A Spell action that is also the full casting time of the spell in question. An action isn’t a pocket of time you get to spend doing things in combat. It is a set of things you can choose one of which to do. Casting a Spell is one of those. Using that action to also make a weapon attack simply means that the weapon attack is part of the spell’s casting.

You are the one twisting RAW because of a misunderstanding of how actions work and what they mean. And it’s a fair misunderstanding, because it’s counterintuitive, especially when we’re used to thinking in terms of spending resources. But your action isn’t something you spend, it’s something you choose to do.

"Using that action to also make a weapon attack simply means that the weapon attack is part of the spell’s casting."

That part i just dont think makes sense - at least to me.

An action doesnt have to be time-based or something you spend to have 2 parts.

Lets say i kick a soccer ball as an "action"
Lets say my coach tells me im not waving my arms enough when i kick the soccer ball, and it'll be a better kick if i wave my arms.

Waving my arms can be part of the action of "kicking a soccer ball" without having anything to do with my foot contacting the soccer ball.


If your interpretation were true, why wouldn't the text read "As part of casting the spell"? The text specifically reads as part of the action.

Having said that, none of that text matters. It doesn't say make the weapon attack from where the spell is cast. It just says, make a weapon attack.

You're trying to blur the lines between the Melee weapon attack and the Cast a spell text to obscure it enough to justify the idea that if the spell is cast from point X, then the melee weapon attack takes place at point X also.

That's a very hefty jump that none of the text supports, and is a much bigger reach than you're giving it credit for.

[Xetheral]

That's the error being made in clearly stated terms.

Invoke Duplicity doesn't change the definition of your current location to match the illusion. It only does it for casting spells. And melee attacks, including the melee attack done as part of the action for Booming Blade, are not casting spells.

I entirely agree that, in the usual case, a melee weapon attack is not part of casting a spell. However, this is not a usual case. Consider:

Normally it is impossible to make a melee weapon attack as part of a Cast a Spell action. Booming Blade and Green Flame Blade both exceptionally permit that to happen. To date, they are the only published material that does so. Ergo, in logical terms, casting one of those two spells is a necessary condition for making a melee weapon attack as part of the Cast a Spell action.

On the other hand, both spells require the caster to make a melee weapon attack as part of the Cast a Spell action, or else the spell fails. Ergo, in logical terms, making a melee weapon attack as part of the Cast a Spell action is a necessary condition for casting those spells.

Since casting either Booming Blade or Green Flame Blade is a necessary condition for making a melee weapon attack as part of a Cast a Spell action, and making a melee weapon attack is a necessary condition for casting either spell, we have a biconditional:

((casting GFB) OR (casting BB)) <-> (making a melee attack as part of a Cast a Spell action)

In colloquial terms, you can't cast BB or GFB without making a melee attack as part of a Cast a Spell action, and you can't make a melee attack as part of a Cast a Spell action without casting BB or GFM.

And yet you're arguing that making a melee attack as part of a Cast a Spell action isn't a part of casting BB or GFB despite their mutual dependency. Can you see why I'm confused?

To use an analogy, to me your position sounds like acknowledging that you can't Use a Wand of Fireballs without expending a charge from the wand. And acknowledging that you can't expend a charge from a Wand of Fireballs without Using the wand. But then claiming that expending a charge from the wand is not part of Using a Wand of Fireballs.

[BarneyBent]

"Using that action to also make a weapon attack simply means that the weapon attack is part of the spell’s casting."

That part i just dont think makes sense - at least to me.

An action doesnt have to be time-based or something you spend to have 2 parts.

Lets say i kick a soccer ball as an "action"
Lets say my coach tells me im not waving my arms enough when i kick the soccer ball, and it'll be a better kick if i wave my arms.

Waving my arms can be part of the action of "kicking a soccer ball" without having anything to do with my foot contacting the soccer ball.


If your interpretation were true, why wouldn't the text read "As part of casting the spell"? The text specifically reads as part of the action.

Having said that, none of that text matters. It doesn't say make the weapon attack from where the spell is cast. It just says, make a weapon attack.

You're trying to blur the lines between the Melee weapon attack and the Cast a spell text to obscure it enough to justify the idea that if the spell is cast from point X, then the melee weapon attack takes place at point X also.

That's a very hefty jump that none of the text supports, and is a much bigger reach than you're giving it credit for.

No, see this is more akin to your soccer coach saying “when you kick the ball, shift your foot position so that when you strike it, you impart some curve on it”. It’s part of the kick action. Just as the attack is a part of the Cast A Spell action. It is that simple.

To continue on your analogy, kicking the ball is the action, waving your hands is not part of kicking the ball, therefore it is not part of the kicking the ball action. It is a separate action. It might be a free action, it might be a bonus action, it might be a reaction, doesn’t matter. It would not be “part of the action used to kick the ball” because the action used to kick the ball IS kicking the ball.

If you want to continue with the waving of hands equaling the weapon attack, the best way to think of it is like this - you have one action you can choose to take, which is to kick the ball with your hands down (let’s call that “Shocking Grasp”). You have another action you can choose to take, which is kicking the ball while waving your hands. Let’s call that Booming Blade. That you can choose to wave your hands with or without kicking the ball does not change the fact that kicking the ball with your hands up is considered, in this strange world with strange rules, it’s own discrete action. So when the description of this action says “using the same action you use to kick the ball”, it means that waving your hands in the air is a part of kicking the ball (specifically, “Kicking The Ball With Your Hands WavingIn The Air”, aka Booming Blade).

From the PHB page 192, Actions In Combat:

When you take your action on your turn, you can take one of the actions presented here, an action you gained from your class or a special feature, or an action that you improvise. Many monsters have action options of their own in their stat blocks.

Notice that the wording is “taking actions” from a list of possibilities. When the wording says “with the same action”, it means the action that you chose to take, not a slot that you own and expended so you could cast a spell.

Now, not all spells have a casting time of 1 action. For those that do, casting the spell is the action you take. If there is something additional as part of that action you take, that means that additional thing is part of casting the spell, NOT that it is separate from casting the spell, and that casting the spell has become less than 1 action (to make room for the other action) all of a sudden

You are using an intuitive understanding of the rules which in most cases is close enough so as not to matter, and that’s understandable. It was the position I started in before I read the rules closer. But the fact is, the way actions work is that casting the spell is the action. So when the spell says “as part of the action used to cast the spell”, that wording is intended to make it absolutely clear that the attack is not part of a different action (e.g. the Attack action, or a free action).

To summarise: you don’t spend your action on casting a spell (if that was the case, your interpretation would possibly be correct - you would still have “room” in “your action” to add a melee attack after casting the spell, just like waving your hands as you kick the ball, though you still run into the problem of the casting time being one full action).

Instead, you don’t “spend your action” on anything. You choose an action to take. So when the wording says “the action used to cast the spell”, they mean the Cast A Spell action. The weapon attack is therefore part of the Cast A Spell action, and therefore part of the spell’s casting. This interpretation is subsequently confirmed by official Sage Advice.

Now, you can rule that that’s silly. That’s fine. I sort of agree. I also think it’s silly casting Shocking Grasp through Invoke Duplicity - the spell explicitly says you touch the target, how do you do that at range?

But once you understand that you don’t use your action to do things, but that you choose an action to take, the wording makes more sense, and the official Sage Advice that explicitly describes the weapon attack as part of the spell’s casting also makes sense.

[BarneyBent]

I entirely agree that, in the usual case, a melee weapon attack is not part of casting a spell. However, this is not a usual case. Consider:

Normally it is impossible to make a melee weapon attack as part of a Cast a Spell action. Booming Blade and Green Flame Blade both exceptionally permit that to happen. To date, they are the only published material that does so. Ergo, in logical terms, casting one of those two spells is a necessary condition for making a melee weapon attack as part of the Cast a Spell action.

On the other hand, both spells require the caster to make a melee weapon attack as part of the Cast a Spell action, or else the spell fails. Ergo, in logical terms, making a melee weapon attack as part of the Cast a Spell action is a necessary condition for casting those spells.

Since casting either Booming Blade or Green Flame Blade is a necessary condition for making a melee weapon attack as part of a Cast a Spell action, and making a melee weapon attack is a necessary condition for casting either spell, we have a biconditional:

((casting GFB) OR (casting BB)) <-> (making a melee attack as part of a Cast a Spell action)

In colloquial terms, you can't cast BB or GFB without making a melee attack as part of a Cast a Spell action, and you can't make a melee attack as part of a Cast a Spell action without casting BB or GFM.

And yet you're arguing that making a melee attack as part of a Cast a Spell action isn't a part of casting BB or GFB despite their mutual dependency. Can you see why I'm confused?

To use an analogy, to me your position sounds like acknowledging that you can't Use a Wand of Fireballs without expending a charge from the wand. And acknowledging that you can't expend a charge from a Wand of Fireballs without Using the wand. But then claiming that expending a charge from the wand is not part of Using a Wand of Fireballs.

God dammit that last paragraph is a better and more succinct explanation than anything I’ve come up with this entire thread. Thank you.

[Xetheral]

God dammit that last paragraph is a better and more succinct explanation than anything I’ve come up with this entire thread. Thank you.

If it makes you feel better, that was like my fifth try at finding a suitable analogy. In this type of debate where both sides use nearly-identical arguments to support diametrically-opposed conclusions I find it can be really tricky to find the right words to talk about the differences. Maybe the analogy will help.

[patchyman]

From my reading what you do as part of casting a spell action is casting a spell and all the parts of this action is the casting of the spell.

If the attack is part of the casting a spell action how could it not be a part of the spell?

Not to open another can of hornets here, but the same way that if you cast the Suggestion spell subtly, you still have to vocalize the suggestion, since Subtle doesn’t make you telepathic.

[BarneyBent]

Not to open another can of hornets here, but the same way that if you cast the Suggestion spell subtly, you still have to vocalize the suggestion, since Subtle doesn’t make you telepathic.

I’m away from books atm but I would think that depends on whether you rule the verbal component IS the utterance, or whether the verbal component is in addition to the utterance.

If the verbal component is the utterance then you’ve got a good argument that yes, your suggestion has been communicated without verbalisation. Which honestly makes just as much sense as delivering Shocking Grasp at 30ft via Distant metamagic. Metamagic makes a lot of the fluff weird.

[Chronos]

Spell Sniper changes the range of the spell. But it does not change the range of the weapon you use with the spell. In exactly the same way, Invoke Duplicity changes your position for purposes of casting the spell. But it does not change your position for purposes of making attacks with your weapon.

The spell descriptions for GFB and BB recognize that there will be some situations where you are able to cast the spell, but not able to attack, and so the spell does nothing. Invoke Duplicity is one of those situations.

Or to take a completely different sort of argument: If you rule that it does work, how? What is actually hitting the target? Are they getting hit by the illusion of a sword in the illusion's illusory hands? How does that do real damage? Are they getting hit by the real sword in your hands, far away from the illusion? How is that real sword getting there? Yes, yes, of course, it's getting there "by magic", but that doesn't answer the question: Is it magically teleporting there? Magically flying through the air? And what's providing the magic to make this happen? Teleporting objects (or making them fly) isn't something that either Booming Blade nor Invoke Duplicity can do.

[tieren]

Or to take a completely different sort of argument: If you rule that it does work, how? What is actually hitting the target? Are they getting hit by the illusion of a sword in the illusion's illusory hands? How does that do real damage? Are they getting hit by the real sword in your hands, far away from the illusion? How is that real sword getting there? Yes, yes, of course, it's getting there "by magic", but that doesn't answer the question: Is it magically teleporting there? Magically flying through the air? And what's providing the magic to make this happen? Teleporting objects (or making them fly) isn't something that either Booming Blade nor Invoke Duplicity can do.

I don't think anyone is arguing the weapon damage would apply, just that swinging the illusionary weapon would be sufficient to complete the casting of the cantrip and do the thunder/fire damage.

[Maelynn]

just that swinging the illusionary weapon would be sufficient

Swinging a weapon isn't an attack - it's not an attack unless you roll a d20 for it (PHB p194). The weapon is physically out of range, so it can't attack. The illusion is merely an image and can't attack. And Invoke Duplicity doesn't state that you're allowed to make weapon attack rolls as if in the illusion's space, so it doesn't grant you the roll that way either.

You can't roll a d20. So you don't make an attack. So the spell fails.

Edit: furthermore, the spells states 'on a hit', so you don't only need to attack, you need to hit. An illusion is incorporeal, so it can't hit physically.

[GooeyChewie]

The spell descriptions for GFB and BB recognize that there will be some situations where you are able to cast the spell, but not able to attack, and so the spell does nothing. Invoke Duplicity is one of those situations.

I would rephrase that first sentence to say that the spell descriptions for GFB and BB recognize that there will be some situations in which you could meet the range, V and M requirements but cannot meet the melee attack requirement, and in those cases the spell fails just as it would if you could not meet another requirements. The fact that such situations exist does not mean that Invoke Duplicity is necessarily one of those situations. If the melee attack is part of the spell (and I believe it is, because I think “is part of the same action as the spell but is not part of the spell” is too fine a line to draw), then you get to make that attack as though you are in the Invoke Duplicity illusion’s location. If nothing else prevents you from making the attack, and the target would be in reach based on ID’s location, then you can make the attack.

Yes, yes, of course, it's getting there "by magic", but that doesn't answer the question:

Yes, it does. Magic is how Guiding Bolt can hit targets 240 feet away from me (but only 120 feet away from my illusion). Magic is how my illusion can heal somebody with Healing Touch even when I can’t touch them. Magic is how spells with M components can be cast over there while the materials are over here by me. Magic is how spells with S components can be cast over there even when my illusion has no mass in its waving arms. The whole point of this portion of Invoke Duplicity is that it allows the illusion to do things illusions can’t logically do.

Edit: Thought of one more. Magic is how I can cast spells with V components when I am in an area affected by Silence and my illusion is not.

I don't think anyone is arguing the weapon damage would apply, just that swinging the illusionary weapon would be sufficient to complete the casting of the cantrip and do the thunder/fire damage.

I am. Either the melee attack is part of the spell and you get to perform it as normal as though you (verbal and material weapon components and all) are in the illusion’s location, or it isn’t part of the spell and the whole thing fails.

And Invoke Duplicity doesn't state that you're allowed to make weapon attack rolls as if in the illusion's space, so it doesn't grant you the roll that way either.

Invoke Duplicity doesn’t list out all the different possibilities within spells. It covers everything within a spell with the specific exception of using your senses. Just because a spell does something that no other spells do doesn’t mean Invoke Duplicity doesn’t let you do it.

[Maelynn]

Invoke Duplicity doesn’t list out all the different possibilities within spells. It covers everything within a spell with the specific exception of using your senses. Just because a spell does something that no other spells do doesn’t mean Invoke Duplicity doesn’t let you do it.

So tell me... how does one make a weapon attack without using their senses?

[BloodSnake'sCha]

So tell me... how does one make a weapon attack without using their senses?

It can be argue that you can't do a thing without using your senses.

You can do it just how you can swing your hand with a sword when your eyes closed ears blocked,your nervs on your skin damaged, your nose block and your moth closed.

You will have hard time to do it but you will.
And I think that seeing the target I cast a spell on from the far side of the room is using my senses.

[Chronos]

Quoth tieren:

I don't think anyone is arguing the weapon damage would apply, just that swinging the illusionary weapon would be sufficient to complete the casting of the cantrip and do the thunder/fire damage.

Do what thunder/fire damage, to whom? That damage is applied to the creature who is hit by the weapon. If there's nobody hit by the weapon, then there's no thunder/fire damage.

Quoth GooeyChewie:

Yes, it does. Magic is how Guiding Bolt can hit targets 240 feet away from me (but only 120 feet away from my illusion). Magic is how my illusion can heal somebody with Healing Touch even when I can’t touch them. Magic is how spells with M components can be cast over there while the materials are over here by me. Magic is how spells with S components can be cast over there even when my illusion has no mass in its waving arms. The whole point of this portion of Invoke Duplicity is that it allows the illusion to do things illusions can’t logically do.

Your Guiding Bolt hits by moving 120' from the illusion's location to the target's location. Your material components work by being in your hands. Your spell with verbal components works by those components being spoken by your illusion. Again, I ask, how does the magic work with the sword?

[GooeyChewie]

So tell me... how does one make a weapon attack without using their senses?

Invoke Duplicity does not eliminate your senses. Your senses are just the one thing it calls out as being measured from your original space instead of the illusion’s space. If you can see the target, roll the attack as normal. If not, disadvantage.

Your Guiding Bolt hits by moving 120' from the illusion's location to the target's location. Your material components work by being in your hands. Your spell with verbal components works by those components being spoken by your illusion. Again, I ask, how does the magic work with the sword?

According to your own words, “Your material components work by being in your hands.” For GFB and BB, the weapon IS listed as the material component. Just like any other material component, Invoke Duplicity allows the sword to work as though you are in the illusion’s space. It’s no different than casting, say Warding Bond from your illusion’s space. For the purposes of the spell, the platinum rings are in that space and you can cast the spell with your ring and the one your friend is wearing.

In other words, magic.

[Maelynn]

Your senses are just the one thing it calls out as being measured from your original space instead of the illusion’s space. If you can see the target, roll the attack as normal. If not, disadvantage.

So you're saying that the weapon attack needs to be measured from your original space, because you need to see the target. The fact that you mention getting a disadvantage only enforces the notion that senses are a necessary aspect for a weapon attack. Well, there we are then. A weapon attack relies on senses. The need for senses dictates using your original space as opposed to the illusion. Your original space is out of melee range. Spell fails.

[GooeyChewie]

So you're saying that the weapon attack needs to be measured from your original space, because you need to see the target. The fact that you mention getting a disadvantage only enforces the notion that senses are a necessary aspect for a weapon attack. Well, there we are then. A weapon attack relies on senses. The need for senses dictates using your original space as opposed to the illusion. Your original space is out of melee range. Spell fails.

No, I did not say the weapon attack needs to be measured from your original space. Just like any other attack as part of a spell, the attack can be measured from the illusion’s space. And just like any other spell, only the determination of whether you can see ( or hear/smell/taste/feel if those matter) the target cares about your original space.

For example, Sacred Flame targets a creature I can see within 60 feet. If I cast through my illusion with ID, I still have to be able to see the creature from my space, but I can measure the 60 feet from my illusion’s space. The need to see the target doesn’t mean I have to measure from my space for that spell, so why would it mean I have to measure from my space for any other spell?

[Tanarii]

Your spell with verbal components works by those components being spoken by your illusion.

Does it? Of the top of my head and without parsing the rules language, I'd think the V, S, and M components are all at the caster.

[BarneyBent]

Spell Sniper changes the range of the spell. But it does not change the range of the weapon you use with the spell. In exactly the same way, Invoke Duplicity changes your position for purposes of casting the spell. But it does not change your position for purposes of making attacks with your weapon.

Spell Sniper explicitly changes the range of the spell. Invoke Duplicity changes your position for the purposes of casting the spell. They are not exactly the same, they are substantially different.

The range is one part of a spell’s casting only. Invoke Duplicity doesn’t actually change anything about the spell - only where you are considered to be as you cast it. And casting it involves making a melee attack.

To contrast the two, imagine an enemy is 10 ft away. If you use Spell Sniper, you have increased the range FROM YOUR CURRENT POSITION of valid targets for the spell, but you can’t hit them in that extra range with the melee attack from your current position (unless you have a reach) so the spell fails. Your current position hasn’t moved for the purposes of the spell, you’ve just changed a specific part of the spell (the range of possible targets).

By contrast, Invoke Duplicity is more akin to temporarily transporting 5ft forward and casting the spell. The melee attack is an explicit part of casting the spell (confirmed in Sage Advice and in accordance with how actions work), therefore you are also that 5ft forward for the melee attack. Just as you would be when you reach out and touch somebody to deliver Shocking Grasp or Guidance.

The spell descriptions for GFB and BB recognize that there will be some situations where you are able to cast the spell, but not able to attack, and so the spell does nothing. Invoke Duplicity is one of those situations.

Actually, “the spell fails”, that doesn’t mean you successfully cast it and it went to waste - it means you failed to cast it. Compare spell failure in the PHB in relation to extended casting times:

When you cast a spell with a casting time longer than a single action or reaction, you must spend your action each turn casting the spell, and you must maintain your concentration while you do so (see “Concentration” below). If your concentration is broken, the spell fails, but you don’t expend a spell slot.

“The spell fails” clearly means “you failed to cast the spell”, not “you cast the spell but it was wasted”.

In any case, the melee attack is part of the Cast A Spell action and so clearly is also modified by Invoke Duplicity, but even if this was more ambiguous, the wording “the spell failed” clearly indicates the weapon attack is a part of the spell’s casting and NOT something you get to do after casting the spell, further strengthening the case that Invoke Duplicity affects it.

Or to take a completely different sort of argument: If you rule that it does work, how? What is actually hitting the target? Are they getting hit by the illusion of a sword in the illusion's illusory hands? How does that do real damage? Are they getting hit by the real sword in your hands, far away from the illusion? How is that real sword getting there? Yes, yes, of course, it's getting there "by magic", but that doesn't answer the question: Is it magically teleporting there? Magically flying through the air? And what's providing the magic to make this happen? Teleporting objects (or making them fly) isn't something that either Booming Blade nor Invoke Duplicity can do.

How does Shocking Grasp work? You explicitly reach out and grab the target. The illusion can’t do that.

Remember, Invoke Duplicity doesn’t mean the duplicate delivers the spell. You cast the spell AS IF YOU WERE IN THE DUPLICATE’S SPACE. This is very specific and unusual wording that differs from, for example, how Familiars deliver spells, how Distant metamagic changes touch spells, etc.

You are, for all intents and purposes, in the duplicate’s space. How that works is fluff, but any fluff you choose to resolve Shocking Grasp can equally apply to Booming Blade. The duplicate is no more capable of touching things than swinging a sword, and it is entirely irrelevant anyway because the whole point is the duplicate does nothing. It can stand stock still, stand on its head, do jumping jacks, whatever, while you cast and deliver the spell, whatever it may be.

[BarneyBent]

I don't think anyone is arguing the weapon damage would apply, just that swinging the illusionary weapon would be sufficient to complete the casting of the cantrip and do the thunder/fire damage.

RAW, I am arguing this. Realistically a DM could argue otherwise and that would be fine. But the illusion doesn’t swing the sword or do anything at all. You just use its position. You swing the sword, you cast the spell, you do all of those, and through the magic of the Trickster God you worship, they all get to affect a target within 5ft of your duplicate instead of you.

How that works is fluff, but it’s absolutely in accordance with RAW because of the specific wording of Invoke Duplicity and the fact that the weapon attack is part of casting the spell.

[BarneyBent]

Your spell with verbal components works by those components being spoken by your illusion.

Your duplicate does no such thing. You say the words, you hold the material components, you make the somatic gestures. You do all of these things “as though you were in the duplicate’s space”. That includes making the melee attack, because that’s explicitly, unambiguously a part of casting the spell.

[RSP]

The illusion is merely an image and can't attack.

This isn’t necessarily true. Invoke Duplicity states it creates “a perfect illusion”. Unfortunately, that’s all we have to go on, but “perfect” is a very strong word in terms of describing something.

A perfect illusion should be one without flaw: no way to detect that it’s an illusion, including through touching it. There are other illusion spells that have substance (Mirage Arcana) and can attack (Shadow Blade and Illusory Dragon both come to mind), therefore, it’s reasonable that a “perfect” illusion wouldn’t be a lesser illusion.

This would mean that the illusion could very well attack, and hit, with an illusory sword.

[Xetheral]

Or to take a completely different sort of argument: If you rule that it does work, how? What is actually hitting the target? Are they getting hit by the illusion of a sword in the illusion's illusory hands? How does that do real damage? Are they getting hit by the real sword in your hands, far away from the illusion? How is that real sword getting there? Yes, yes, of course, it's getting there "by magic", but that doesn't answer the question: Is it magically teleporting there? Magically flying through the air? And what's providing the magic to make this happen? Teleporting objects (or making them fly) isn't something that either Booming Blade nor Invoke Duplicity can do.

(Emphasis added.) I agree with the bolded proposition. The attack is made at the caster's current location with an actual weapon, and, due to Invoke Duplicity, affects the target "as though" the caster was instead at the location of the duplicate. The weapon does not need to teleport or otherwise move--the cause and the effect are simply not co-located in space.

In the game world, it would look a lot like the visual depiction of the fight between Gandalf and Saruman in Fellowship of the Ring: when each wizard swings their staff, and the other wizard is affected by the blow even though the wizards are out of reach from each other. The major difference is that in D&D, there's also an illusory duplicate (who may or may not appear to be doing anything, depending on the specific table's interpretation of Invoke Duplicity).

[JackPhoenix]

(Emphasis added.) I agree with the bolded proposition. The attack is made at the caster's current location with an actual weapon, and, due to Invoke Duplicity, affects the target "as though" the caster was instead at the location of the duplicate. The weapon does not need to teleport or otherwise move--the cause and the effect are simply not co-located in space.

In the game world, it would look a lot like the visual depiction of the fight between Gandalf and Saruman in Fellowship of the Ring: when each wizard swings their staff, and the other wizard is affected by the blow even though the wizards are out of reach from each other. The major difference is that in D&D, there's also an illusory duplicate (who may or may not appear to be doing anything, depending on the specific table's interpretation of Invoke Duplicity).

The attack is made at the caster's location with actual weapon, and Invoke Duplicity does nothing, because it only affects spells, not weapon attacks. If the target isn't in the weapon's reach, the cantrip will fail, per its description.

In the game world, it would look like the caster swinging their weapon at empty air, looking like a fool in the process. Gandalf and Saruman used telekinesis to throw each other around, the staves served as foci, their weren't hitting each other with them from 20' away.

[Maelynn]

The illusion is merely an image and can't attack.

This isn’t necessarily true. Invoke Duplicity states it creates “a perfect illusion”. Unfortunately, that’s all we have to go on, but “perfect” is a very strong word in terms of describing something.

A perfect illusion should be one without flaw: no way to detect that it’s an illusion, including through touching it. There are other illusion spells that have substance (Mirage Arcana) and can attack (Shadow Blade and Illusory Dragon both come to mind), therefore, it’s reasonable that a “perfect” illusion wouldn’t be a lesser illusion.

This would mean that the illusion could very well attack, and hit, with an illusory sword.

This is definitely not RAW, but just an attempt to twist the definition of an illusion. An illusion is not corporeal. PHB p203 explains that illusions are images/sounds/memories that deceive the senses, but are still phantoms. They cannot affect the world because everything passes through it. Thinking that 'perfect' means it becomes corporeal is an absurd interpretation, because nowhere does it say that it does. Check the 14th level ability of the Illusion Wizard - it allows them to make 1 illusion real enough to interact with, but it still can't damage or harm anyone. That's the only illusion that can gain some form of corporealness, and only because the ability explicitly states that it can.

Even if, for the sake of argument, we could state that you moving your weapon allows the illusion to move theirs, it still wouldn't hit. And hitting the target is a crucial part of the BB/GFB spells. It can't hit, so the spell fails.

[RSP]

This is definitely not RAW, but just an attempt to twist the definition of an illusion. An illusion is not corporeal. PHB p203 explains that illusions are images/sounds/memories that deceive the senses, but are still phantoms. They cannot affect the world because everything passes through it. Thinking that 'perfect' means it becomes corporeal is an absurd interpretation, because nowhere does it say that it does. Check the 14th level ability of the Illusion Wizard - it allows them to make 1 illusion real enough to interact with, but it still can't damage or harm anyone. That's the only illusion that can gain some form of corporealness, and only because the ability explicitly states that it can.

RAW, Invoke Duplicity is “a perfect illusion.” That’s the definition we get. Saying that’s not RAW, is incorrect.

Saying interpretations of that “is definitely not RAW”, is also incorrect, if those interpretations do not otherwise violate RAW.

Mirage Arcana allows for solids to effectively become liquids, liquids become solids, illusory structures to have substance, etc. So we can safely say illusions are capable of such.

Illusory Dragon states “The illusion lasts for the spell’s duration and occupies its space, as if it were a creature.” So we can safely say an illusion can occupy a space, just like creatures can, meaning it can’t be passed through by enemies, but could be shoved.

Further, the Illusory Dragon can create real fire, cold, acid, poison, lightning: “At any point during its movement, you can cause it to exhale a blast of energy in a 60-foot cone originating from its space. When you create the dragon, choose a damage type: acid, cold, fire, lightning, necrotic, or poison. Each creature in the cone must make an Intelligence saving throw, taking '7d6 damage of the chosen damage type on a failed save, or half as much damage on a successful one.”

Note: not psychic damage, but real damage from the chosen energy type; so real fire, real acid, whatever.

Shadow Blade creates a real weapon. Note, this weapon can be used to parry and block other weapons, such as with the Defensive Duelist Feat. So the illusion is solid. Also, it’s a weapon that can make attacks and cause damage.

So if illusions are capable of having mass, occupying space, being weapons, making attacks, causing damage; why wouldn’t “a perfect illusion” be capable of such?

Put another way, any illusion incapable of doing what other illusions can clearly do would be, by definition, imperfect.

So, you’re stating Invoke Duplicity is creating an imperfect illusion, but the RAW clearly states Invoke Duplicity creates “a perfect illusion.” So, in this sense, you’re actually the one that would be going against RAW.

[BarneyBent]

The attack is made at the caster's location with actual weapon, and Invoke Duplicity does nothing, because it only affects spells, not weapon attacks. If the target isn't in the weapon's reach, the cantrip will fail, per its description.

In the game world, it would look like the caster swinging their weapon at empty air, looking like a fool in the process. Gandalf and Saruman used telekinesis to throw each other around, the staves served as foci, their weren't hitting each other with them from 20' away.

The weapon attack is part of the spell. Therefore, Invoke Duplicity affects it. It is really that simple.

[JackPhoenix]

The weapon attack is part of the spell. Therefore, Invoke Duplicity affects it. It is really that simple.

Indeed, it's simple. Invoke Duplicity allows you to make weapon attack against a target within 5' of the duplicate, rather than within 5' of yourself, as part of the action used to cast the spell. However, if that target is outside the reach of the weapon, which is located at your actual position, you can't make the attack, and the spell will fail.