Phantasmal Force Question

[Creyzi4j]

Can phantasmal force cause conditions such as blindness and deafness?

Like if I use phantasmal force to create a regenrating flaming facehugger with spikes that keeps on leaping at the targets face even if the target removes it will the target be..

Blinded?

Other conditions like creating giant leeches on a creature's facw

[Millstone85]

I would say yes.

However, I am also of the opinion that interactions with the phantasm should be resolved as if it were real.

If the target thinks some alien thing is trying to hug their face, the target can imagine the attempt failing. At the game table, that's a grapple attack.

The target sees the thing bleed and heal? At the game table, the facehugger has hit points and some feature that restores them.

And the target might manage to kill the creature in their mind, or throw it out a window, before they realize it is a phantasm.

[Creyzi4j]

What if the face hugger appears directly at his face?

That would be one wasted action for the enemy right?
Pretty nice, I think.

One wasted axn is better than dmg sometimes. IOW, i think that would be pretty balanced for a level 2 spell

[jas61292]

This is a question that has come up before, with no cast consensus. Personally, I'd say no. The spell does not say it can cause conditions, and so to do so goes beyond the extent of the spell. The subject will rationalize the result, but rationalization means altering their thoughts on what happened or is happening, not altering their actual physical perception. As the spell cannot cause blindness, attempting to do so cannot work. It might mean the person think they are blinded, but somehow developed a sixth sense that lets them "see" as well as ever. Or maybe to them, the thing on their face is transparent. But it will not blind them. It simply cannot do that.

[MarkVIIIMarc]

My vote is if the DM says so.

If it were me, I'd take into consideration:

Give the opponent a reason to poke their eyes out.

Make it a low wisdom opponent who rolls REALLY poorly on saves, for a long time.

Cast Phantasmal Force using a higher level slot.

[Lombra]

Phantasmal force a hood that strangles someone, and he's not gonna see through the hood. I think I read it in a Sage Advice, can't provide a link at the momento tho.

[SharkForce]

i would say a phantasmal force can obscure senses(for example, i would say you could make an illusion of a bag over the target's head), but cannot cause lasting conditions (you cannot stab their eyes to blind them).

[Desteplo]

i would say a phantasmal force can obscure senses(for example, i would say you could make an illusion of a bag over the target's head), but cannot cause lasting conditions (you cannot stab their eyes to blind them).

Though they might close their eyes because they think they're being stabbed. No person is going to keep their eyes open for that. In that case blinded

[Creyzi4j]

This is a question that has come up before, with no cast consensus. Personally, I'd say no. The spell does not say it can cause conditions, and so to do so goes beyond the extent of the spell. The subject will rationalize the result, but rationalization means altering their thoughts on what happened or is happening, not altering their actual physical perception. As the spell cannot cause blindness, attempting to do so cannot work. It might mean the person think they are blinded, but somehow developed a sixth sense that lets them "see" as well as ever. Or maybe to them, the thing on their face is transparent. But it will not blind them. It simply cannot do that.

. It rationalizes the illogical outcome. It's probably a safety word put thwre by the developers to give the DM some say if players every use the spell to create massive condixn creation.
I've always thought the rationalize part as given to the DM to stop player abuses with the spell. Players tend to focus on the "rationalize" word in the spell description instead of the "illogical outcome" word.

For example creating an illusion of a chain to restrain someone. The "illogical outcome" would be, "why am I not restrained?". The rationalize part would then be "the chain must jave been very rusty and weak" so I didn't need to effort any attempt to get out of the restraint.

Or creating tentacles to knock someone prone. The illogical outcome would be "why am I not prone?". The "rationalize" part would be. "It's a weak tentacle, and I brush it off as minor annoyance"

Instead of viewing the "rationalize" word in the spell description to abuse the spell, it should be viewed as a word to avoid abuses.

However, the idea of blocking the view of someone using illusions should be taken into account since illusions focus a lot of sight.

Also, I think blinded would be the wrong word for it. But using the illusion to block the creature's view. It gives similar results to the blinded condixn

[SharkForce]

. It rationalizes the illogical outcome. It's probably a safety word put thwre by the developers to give the DM some say if players every use the spell to create massive condixn creation.
I've always thought the rationalize part as given to the DM to stop player abuses with the spell. Players tend to focus on the "rationalize" word in the spell description instead of the "illogical outcome" word.

For example creating an illusion of a chain to restrain someone. The "illogical outcome" would be, "why am I not restrained?". The rationalize part would then be "the chain must jave been very rusty and weak" so I didn't need to effort any attempt to get out of the restraint.

Or creating tentacles to knock someone prone. The illogical outcome would be "why am I not prone?". The "rationalize" part would be. "It's a weak tentacle, and I brush it off as minor annoyance"

Instead of viewing the "rationalize" word in the spell description to abuse the spell, it should be viewed as a word to avoid abuses.

However, the idea of blocking the view of someone using illusions should be taken into account since illusions focus a lot of sight.

Also, I think blinded would be the wrong word for it. But using the illusion to block the creature's view. It gives similar results to the blinded condixn

you could interpret it that way. but then you may as well just cross it out entirely and ban it from your games, because that would make it a trap option, and the game is better off with it not existing at all. which, to be fair, appears to be how the spell works, based on tweets, so i have no idea why they put it in the book in the first place.

[Millstone85]

Instead of viewing the "rationalize" word in the spell description to abuse the spell, it should be viewed as a word to avoid abuses.

You should appreciate that people are trying to find uses for the spell, without immediately calling those abuses.

However, the idea of blocking the view of someone using illusions should be taken into account since illusions focus a lot of sight.

The phantasm is a rarity in that regard, as it provides "sound, temperature, and other stimuli" in addition to being visible.

For example creating an illusion of a chain to restrain someone. The "illogical outcome" would be, "why am I not restrained?". The rationalize part would then be "the chain must jave been very rusty and weak" so I didn't need to effort any attempt to get out of the restraint.

Here, I would say the chain should appear very restraining, including by its weight and pressure on your wrist, at least until you try to break it or force your hand free, at which point it may indeed reveal itself rusty and weak.

[Gignere]

I don't understand why people think there should be ways to rationalize out of Phantasmal Force besides RAW, everything about realizing the illogical outcomes of the stimuli or phantasm is baked into the spell as the investigation check. Don't want to be fooled by phantasmal force stop dumping int.

[jas61292]

I don't understand why people think there should be ways to rationalize out of Phantasmal Force besides RAW, everything about realizing the illogical outcomes of the stimuli or phantasm is baked into the spell as the investigation check. Don't want to be fooled by phantasmal force stop dumping int.

Because people misunderstand the word "rationalize." To rationalize is to "attempt to explain or justify with logical, plausible reasons, even if these are not true or appropriate." The spell makes them perceive something that is not there. That thing cannot actually effect them in any way, except to cause some psychic damage, if appropriate, but they perceive it. Saying that it should have greater effects because "that's what that object would logically do and they rationalize it to be that way" is a fundamental misunderstanding of the word.

As was said a couple posts ago, rationalizing ropes that are tied around you could be done by thinking "huh, those were weak ropes, and getting out of them was easy." It is not going "oh no, there are ropes around me; I must be restrained," because that is adjusting the reality to the reasoning, not the reasoning to the reality.

Phantasmal Force is a very good spell because they will rationalize it away. I have see it used to distract individuals by creating an additional "combatant" who chips away at an enemy while "dodging" every attack thrown at them. I have seen it used to create obstacles that opponents spend time going around rather than fighting. And I have seen many other great uses. Just because you can't turn it into an infinitely versatile thing doesn't mean it is not fantastic.

[Coidzor]

My vote is if the DM says so.

If it were me, I'd take into consideration:

Give the opponent a reason to poke their eyes out.

Make it a low wisdom opponent who rolls REALLY poorly on saves, for a long time.

Cast Phantasmal Force using a higher level slot.

Phantasmal Force targets the rare Intelligence Saving Throw. Then on subsequent rounds, they don't get a save, they have to decide if they want to use their Action to roll Intelligence (Investigation) checks to try to get out of it.

A convincing enough illusion or a pressing enough distraction will keep many stupid enemies(who you'd predominantly use this on) from ever attempting an Investigation roll.

Or maybe to them, the thing on their face is transparent.

That cannot be the case. You only see through illusions if you successfully disbelieve them or you are the one that created the illusion in the first place. Don't break actual RAW when trying to work out an interpretation of a spell. Especially if you're going to come out with the tone and ethos that you did in your post.

Just because you can't turn it into an infinitely versatile thing doesn't mean it is not fantastic.

Well, unless you get a hostile DM like Millstone85 who will actively try to nerf it by saying that they can just up and kill the illusion.

Illusion spells are completely worthless with a DM like that, OK to moderately good with a more conservative DM like you, and then the most powerful thing short of Wish and the like if full creativity is allowed with them.

[Pex]

Well, unless you get a hostile DM like Millstone85 who will actively try to nerf it by saying that they can just up and kill the illusion.

Illusion spells are completely worthless with a DM like that, OK to moderately good with a more conservative DM like you, and then the most powerful thing short of Wish and the like if full creativity is allowed with them.

Generally this. I view "hostile" DMs, to use Coidzor's meaning not my usual meaning of tyrannical DMing, as metagaming. The DM knows the spell effect is an illusion, so the NPC will automatically be circumspect of the situation. The NPC may or may not automatically know it is an illusion, and this goes for any illusion spell not just Phantasmal Force, but the NPC will not treat it as normal. At best you get a round of the NPC not doing anything against the party before ignoring it. Cast real Wall of Stone, the NPC is blocked and has to walk around if able. Cast Silent Image of a stone wall, NPC automatically gets a saving throw to disbelieve it.* If he makes the save he walks on through. The NPC of a "hostile" DM would never just believe it's a real wall of stone to be blocked or walk around. He always has a chance of disbelieving it. The DM will always try to give the NPC a reason why he would try to disbelieve whatever illusion a player creates, never just accepting the situation presented. Such DMs might as well just ban all illusion spells and get it over with.

*Some DMs wouldn't even be that generous. The DM would have the NPC touch the wall, realize it's an illusion, and walk on through with no saving throw at all completely wasting your spell. If it was a real Wall of Stone, the NPC would not have tried to touch it and just accept it was real to be blocked or walk around.

[SharkForce]

Because people misunderstand the word "rationalize." To rationalize is to "attempt to explain or justify with logical, plausible reasons, even if these are not true or appropriate." The spell makes them perceive something that is not there. That thing cannot actually effect them in any way, except to cause some psychic damage, if appropriate, but they perceive it. Saying that it should have greater effects because "that's what that object would logically do and they rationalize it to be that way" is a fundamental misunderstanding of the word.

As was said a couple posts ago, rationalizing ropes that are tied around you could be done by thinking "huh, those were weak ropes, and getting out of them was easy." It is not going "oh no, there are ropes around me; I must be restrained," because that is adjusting the reality to the reasoning, not the reasoning to the reality.

Phantasmal Force is a very good spell because they will rationalize it away. I have see it used to distract individuals by creating an additional "combatant" who chips away at an enemy while "dodging" every attack thrown at them. I have seen it used to create obstacles that opponents spend time going around rather than fighting. And I have seen many other great uses. Just because you can't turn it into an infinitely versatile thing doesn't mean it is not fantastic.

you mean, until they realize there's a hard-to-hit non-threatening enemy and walk away, and it turns out that phantasmal force is entirely stuck within a 10 foot cube and can't do a damned thing? or until they rationalize that they just smashed through the obstacle when they tried to climb it and just popped out the other side?

if you can just do whatever you want, and rationalize that it was supposed to happen, phantasmal force does nothing except eat someone's spell slot and concentration for no value. if you're going to do that to it, you should just remove it entirely, because at that point it is just a trap option with no value.

[scalyfreak]

you mean, until they realize there's a hard-to-hit non-threatening enemy and walk away, and it turns out that phantasmal force is entirely stuck within a 10 foot cube and can't do a damned thing? or until they rationalize that they just smashed through the obstacle when they tried to climb it and just popped out the other side?

Unless I misunderstood the spell description, the hard-to-hit Phantasmal Fighter can cause psychic damage. And if not, the party rogue can do sneak attacks against the spell target now.

[jas61292]

you mean, until they realize there's a hard-to-hit non-threatening enemy and walk away, and it turns out that phantasmal force is entirely stuck within a 10 foot cube and can't do a damned thing? or until they rationalize that they just smashed through the obstacle when they tried to climb it and just popped out the other side?

if you can just do whatever you want, and rationalize that it was supposed to happen, phantasmal force does nothing except eat someone's spell slot and concentration for no value. if you're going to do that to it, you should just remove it entirely, because at that point it is just a trap option with no value.

You can just ignore it, sure. But unless you have metagame reasons for doing so, you really can't. In the case of the obstacle, if you make it something weak or easy to climb, yeah they will try to break it or climb it and get by it easily. On the other hand if you make it a wall of fire, they will likely go around, as it looks and feels like a wall of fire, and unless they are dumb (or fire resistant) they likely won't try it. The reason things like ropes or whatnot are a bad use is because they will try to interact with them and then have no problems when, if it were real, they would.

The best ways to use it are to create things that the target will not find odd, so they never think to investigate, as well as things they will not directly interact with because they are dangerous or pointless to interact with, or things they want to be able to interact with, but won't be able to. Sure, an illusory creature won't be hitting that hard, but against stupider opponents, it can tie them up in "melee" with a non existent opponent.

But if your idea is to try and make them think something is effecting them when it is not, this spell simply won't work, because it cannot actually cause things to happen beyond the illusion itself. Why it doesn't function as it should in real life will be rationalized away, and it will be worthless. Trying to have people react as if something that is not there is actually effecting them (other than with minor damage) is beyond this spell, and frankly beyond this entire school of magic. This is an illusion, and what people want out of it is an enchantment. But that does not work.

[SharkForce]

Unless I misunderstood the spell description, the hard-to-hit Phantasmal Fighter can cause psychic damage. And if not, the party rogue can do sneak attacks against the spell target now.

You can just ignore it, sure. But unless you have metagame reasons for doing so, you really can't. In the case of the obstacle, if you make it something weak or easy to climb, yeah they will try to break it or climb it and get by it easily. On the other hand if you make it a wall of fire, they will likely go around, as it looks and feels like a wall of fire, and unless they are dumb (or fire resistant) they likely won't try it. The reason things like ropes or whatnot are a bad use is because they will try to interact with them and then have no problems when, if it were real, they would.

The best ways to use it are to create things that the target will not find odd, so they never think to investigate, as well as things they will not directly interact with because they are dangerous or pointless to interact with, or things they want to be able to interact with, but won't be able to. Sure, an illusory creature won't be hitting that hard, but against stupider opponents, it can tie them up in "melee" with a non existent opponent.

But if your idea is to try and make them think something is effecting them when it is not, this spell simply won't work, because it cannot actually cause things to happen beyond the illusion itself. Why it doesn't function as it should in real life will be rationalized away, and it will be worthless. Trying to have people react as if something that is not there is actually effecting them (other than with minor damage) is beyond this spell, and frankly beyond this entire school of magic. This is an illusion, and what people want out of it is an enchantment. But that does not work.

i ignore threats that do tiny amounts of damage all the time. or at least, i do when there is a much bigger threat elsewhere. i'm not saying the target would be thinking "well, i'll just leave and this thing will eventually disappear". i'm saying that if i'm fighting a creature and it is doing 3.5 damage per round on average, and over there my friends are facing creatures that are dealing far more than 3.5 damage per round, and i am making absolutely no progress in harming the creature i'm fighting... i'm just going to leave that hard-to-hit low-threat creature for last. just like i would do in a fight with real creatures, and just like i expect any monster to do if there was a character in the party who was extremely hard to hit (say, a monk who is dodging) who represents little threat (say, if the creatures have a lot of hit points and excellent con saves) compared to other characters who are both more dangerous and less tough (like, say, that guy over there who is throwing lightning bolts and doesn't seem to be dodging everything that comes his way).

low-threat creatures that are hard to kill can be mopped up at the end of the fight. heck, even low-threat creatures with little toughness are going to take priority over low-threat high-toughness.

or, to put it another way... imagine you're a group of soldiers and you're supposed to attack a location. that location is at the top of the hill, there are four guys with pointy sticks hiding in a bunker, poking people who come near, and one guy manning a heavy machine gun behind a 2 foot high sandbag wall. who is going to be your highest priority to deal with first?

now, the phantasmal force can certainly appear to be something that should be high priority, so if we're talking about one, *maybe* 2 rounds of it being focused, i could see that. but the moment it becomes obvious that it isn't actually doing a lot of damage, i expect it to be ignored. even if it is a wall of fire, unless you're talking about maybe a kobold or something where being hit by a commoner with a club could end their life, i would expect most creatures to be willing to risk a 1d6 damage wall of fire for something important enough.

[Creyzi4j]

So...regarding the facehugger illusion or any illusion blocking LOS issue? It could work? If not, then how would the DM remove it away if one of his creatures get hit.

I mean, the blindness spell is the same level as phantasmal force. It's just that they have their differences.
In terms of balance I think a comparison between the two could take into effect.

Blindness: pros = has no concentraxn, can be cast at higher levels, no components (for component heavy campaigns), it can affect target with true sight
Cons = save per round, it's a con save (very common high stat), shorter range

Phantasmal force: pros = higher range, it's an int save, longer range, no save per round? Has other uses (flexible),
Cons = it's a concentration spell, cannot be cast at higher levels, undeads andnconstructs are immune to it, some monsters immune to it's block view effect "but not to the blindness spell" (true sight)

I think they are pretty much the same. Phantasmal Force on the other hand should be considered more powerful than blindness since it uses concentration.

[ThePolarBear]

[...] and it turns out that phantasmal force is entirely stuck within a 10 foot cube and can't do a damned thing?[...]

10ft cube is the maximum dimension of the illusion itself. In fact the spell itself does not say where the illusion has to be placed - there are no rules about that or about the ability of the illusion itself to be able to move.

In fact, it appears that RAI it is intended for the illusion to be able to both move and leave the original "point of origin"

[Rebonack]

You can make an illusion of a wall and hide behind it with Silent Image. Creatures that haven't interacted with it will be unable to see through it- it blocks their line of sight. So there's already precedent for illusions being able to impede senses. And we're told that Phantasmal Force is able to impose upon ALL senses. Not just sight.

So what do we do with this?

You use Phantasmal Force to conjure up an illusion of a Nightseed, a horrible cloud of malevolent black mist from the Far Realm that has a pretty nasty engulf attack. The Nightseed proceeds to roll over the top of the target who failed the save, plunging them into absolute blackness that burns like acid and shrieks like the depths of Pandemonium. The illusion is impeding two senses, sight and hearing. That should Blind the target and possibly Deafen them, too. Or at the very least impose disadvantage on perception checks relating to hearing.

If we assume that Phantasmal Force is unable to impede senses at all, then it can't block line of sight. If it can't block line of sight then anything it creates is by default totally transparent and obviously illusionary. If everything it creates is obviously illusionary then the spell is totally pointless.

[SharkForce]

10ft cube is the maximum dimension of the illusion itself. In fact the spell itself does not say where the illusion has to be placed - there are no rules about that or about the ability of the illusion itself to be able to move.

In fact, it appears that RAI it is intended for the illusion to be able to both move and leave the original "point of origin"

illusions that can move generally say so. nothing in phantasmal force remotely suggests the illusion can move. in fact, it says it has to fit inside a 10 foot cube. you are correct that it does not specify the required location of the cube, though as it is part of the spell it likely must be in the spell's range. but absent any statement that it can move from that cube, there is no reason to presume it can, any more than there is for any other spell.

i mean, i could argue that nothing says a teleportation circle spell doesn't move if you want it to. it never says it has to stay in its original location. or a wall of fire. it doesn't say it can't move. there is just as much stating that those spells can move from their original location as there is that phantasmal force can move from its original location.

ultimately, the rules cannot reasonably be expected to tell us everything something *can't* do. the only rational presumption is that where a spell does something, it tells you it does something, otherwise almost any spell could be argued to do almost any thing, as it doesn't say they don't do that thing (and if they tried that, each spell could be about as long as an encyclopedia and still wouldn't cover every possible thing).

for example: does magic missile turn you into a level 20 wizard? well, it doesn't say that it can't! does casting polymorph let you gain the full wild shape ability of a moon druid? well, it doesn't say that it can't! does cure wounds instantly defeat all enemies? well, it doesn't say that it can't!

so yeah. that's why phantasmal force doesn't move. if it doesn't say it can move around freely, it's stuck with the initial area. and the initial area for a phantasmal force spell is a 10 foot cube.

[jas61292]

illusions that can move generally say so. nothing in phantasmal force remotely suggests the illusion can move. in fact, it says it has to fit inside a 10 foot cube. you are correct that it does not specify the required location of the cube, though as it is part of the spell it likely must be in the spell's range. but absent any statement that it can move from that cube, there is no reason to presume it can, any more than there is for any other spell.

i mean, i could argue that nothing says a teleportation circle spell doesn't move if you want it to. it never says it has to stay in its original location. or a wall of fire. it doesn't say it can't move. there is just as much stating that those spells can move from their original location as there is that phantasmal force can move from its original location.

ultimately, the rules cannot reasonably be expected to tell us everything something *can't* do. the only rational presumption is that where a spell does something, it tells you it does something, otherwise almost any spell could be argued to do almost any thing, as it doesn't say they don't do that thing (and if they tried that, each spell could be about as long as an encyclopedia and still wouldn't cover every possible thing).

for example: does magic missile turn you into a level 20 wizard? well, it doesn't say that it can't! does casting polymorph let you gain the full wild shape ability of a moon druid? well, it doesn't say that it can't! does cure wounds instantly defeat all enemies? well, it doesn't say that it can't!

so yeah. that's why phantasmal force doesn't move. if it doesn't say it can move around freely, it's stuck with the initial area. and the initial area for a phantasmal force spell is a 10 foot cube.

I'd disagree with that assessment, because nowhere does it say anything about the illusion's location. the 10 ft cube thing is about size and only size. Not location. Furthermore, the illusion inherently has no location in the first place, since this is not a figment in the real world that others see, even though it is not there, but rather a phantasm that only exists inside the mind of the victim. Its location has no physical space at all. Rather, if movement would be logical for the kind of illusion being created, then it would absolutely "move" in the mind of the target.

Claiming that it cannot move because it has a size (not a location) restriction would be like claiming you cannot move the object created by the Creation spell, because it also has a size restriction of a 5 ft cube.

[Rebonack]

illusions that can move generally say so. nothing in phantasmal force remotely suggests the illusion can move.

Sure it does.

It says that it can create a phantasm of a creature and that phantasmal creature can 'attack' the afflicted target. If the creature were unable to move, it would have a difficult time doing this unless it's assaulting the victim with an unpleasant odor. Every other illusion spell that can create illusions of living creatures make illusions that can move, otherwise the spell would be immediately unconvincing. Silent Image requires you to direct the illusionary creature with an action. The phantasm, however, is wholly within the target's mind. Once you cast the phantasmal creature (or object) with its set behaviors it is going to act on its own until your concentration is broken or the target realizes that the creature is just a figment of their own mind.

This is consistent with both the spell description and the Sage Advice clarification.

[SharkForce]

it is not consistent with the spell description. the spell doesn't say the illusion can go anywhere. and it only says the illusion can attack (or otherwise do damage) if it is within 5 feet. it doesn't say it can chase the target. it doesn't say it can go wherever it wants. it doesn't say you have any control over it to make it move around.

just like a whole bunch of other spells that don't say they let you move things around.

if it gives you a 10x10x10 cube, that's what you get. not a mobile 10x10x10 cube. just a plain, ordinary 10x10x10 cube. that's where the illusion is.

[Rebonack]

So we've got here a completely immobile 'creature' that is being created. Nice big life-like statue. Very convincing. However, it is utterly incapable of moving in any way. After all, the spell description says exactly as much about movement as Minor Illusion does. Thus, the creature's attack would consist of the afflicted target being quite convinced that this completely immobile 'creature' hit them. Somehow. Obviously it happened when they blinked or looked away, because they certainly didn't see it happen. Or so the rationalization would go. I think we've made it pretty clear that the spell, if interpreted as strictly as possible, is completely non-functional. You said so yourself already. So maybe, given the absurdity of a highly strict reading, we ought to be a little more flexible with it?

Do you suppose that perhaps some vital information, such as the ability for the illusion to move anywhere within its range, might have been left out of the spell? That might explain why the various Sage Advice answers have consistently ruled that the Phantasm is able to move with the target in the case of dire bags and covering them in spiders. RAW or no, RAI appears to be that the phantasm behaves and moves as a real creature would.

[SharkForce]

oh, the creature can move within the 10 foot cube. it is an illusion of a creature, not an object. but it doesn't say it can go wherever it pleases, so it can't.

[Rebonack]

The spell says nothing about the creature being able to move within the cube. Exactly as much nothing as a Minor Illusion of a gnome mannequin being able to move inside its five foot cube. The spell says nothing about movement within the cube, simply that the object or 'creature' must be smaller than said cube. Case in point, Silent Image only speaks of the image it creates being anything other than static when you're using your action to move its cube.

As the image changes location, you can alter its appearance so that its movements appear natural for the image. For example, if you create an image of a creature and move it, you can alter the image so that it appears to be walking.

It needs to be specified that taking an action causes the image to appear to be moving in a natural way as opposed to sliding across the ground like a poorly mapped Elder Scrolls NPC. The implication here is that when you aren't using your action to move the image, it's just as static as the gnome mannequin.

And by the by, if the creature is well and truly restricted within its cube, how does it attack someone standing within five feet of the cube? Obviously the phantasm of the gelatinous cube must have tentacle slapped the poor target while they weren't looking, as the phantasm can't leave its cube.

I want to point out here that I don't believe that any of these rulings are reasonable. I'm simply trying to show that an overly strict reading of the illusion spells makes them terrifically not-fun. These should be a source for players to flex their imagination and pull off inventive shenanigans, not shackle them with technicalities that make the spells a hassle to utilize. The Sage Advice answers all seem to be in agreement on this one. The phantasm can follow the target. How far? Seems to be up to the DM. The spell could probably use some errata if that's what Wizards wants it to do.

[RSP]

The JC tweet indicates the PF can move and, more to the RAW, the only reference to location (as opposed to size) is: "You craft an illusion that takes root in the mind of a creature that you can see within range."

Per the RAW, the location of the illusion is always where the target is as it's 'rooted in their mind.' If they run, the fire comes after them, or the giant snake, or the armored knight, etc.

Based on this, I'd say the 10' cube is needed to be touching the space of the target, and does, in fact, travel with it.