The mechanics of stabbing someone in the back, literally.

[rmnimoc]

So in a recent game our bard managed to just waltz into an area we had to go through, convinced the people in the area that they worked for the same guy through some fast talking and faster spellcasting, and then got all the guards but one to run off. The one remaining thought that the bard was a childhood friend, and the bard talked the guard into a massage. During this, he literally stabbed her in the back. It was ruled at the table to be an instant crit, and the damage killed her, but it got me wondering about it and I couldn't seem to find any adequate explaination in the books for how to work this.

So, how would you handle a literal backstab to someone with their guard completely down and no reason to suspect or believe that anything might happen at your tables?

[LeonBH]

It would be a surprise round.

Everyone rolls initiative. Then, the first round is a surprise round. The attacker gets one turn in combat while the surprised parties cannot act, move, or react. After the 1st round is done, the 2nd round proceeds normally. There is technically no advantage granted to these attacks because the reward is already the surprise round.

The autocrit and advantage on an attack during a surprise round only applies to the Assassin Rogue. Handing it out carelessly nerfs any assassins in your party by making their 3rd level ability redundant.

[JackPhoenix]

It would be a surprise round.

Everyone rolls initiative. Then, the first round is a surprise round. The attacker gets one turn in combat while the surprised parties cannot act, move, or react. After the 1st round is done, the 2nd round proceeds normally. There is technically no advantage granted to these attacks because the reward is already the surprise round.

The autocrit and advantage on an attack during a surprise round only applies to the Assassin Rogue. Handing it out carelessly nerfs any assassins in your party by making their 3rd level ability redundant.

There's no surprise round in 5e. Surprise is more of a unlisted condition. The surprised creatures can't act, that much is true, but they can use reactions once their initiative order comes to pass... possibly before the bard, if they get better initiative. (technically, it's not surprise by RAW, as the victim was aware of bard's presence, but it's reasonable ruling)

You would get advantage, because the massaged creature propably lies prone. No autocrit unless it's also paralyzed or incapacitated, or the character is assassin rogue who also rolled better initiative.

[Kane0]

By default that's a combat with surprise.

BUT There's a couple factors that may or may not come into play when DMing this sort of thing:

- Is there danger/is the guy a redshirt?
If there's no threat to the PCs it may simply be a waste of time to roll a combat if the guy would die to one stab anyways. That time could be better spent dealing with consequences and moving the game forward.

- Is this more interesting/fun as an 'event' or an 'encounter'?
Certain types of games (intrigue, heroic, cinematic, horror, etc) will color how you want to play it. Sometimes the tone of the game will be unconcerned with the specifics of how a guy gets killed, and more on how loudly or messily for example.

- Do you or the players want to do a combat?
If you have just finished or are moving towards a big fight, or the session has had plenty of combat already, the DM might just want to say the guy is stabbed and that's that. Breaking the flow might just be a bad idea.

At the end of the day, not every bit of bloodshed needs to be done by the combat rules, that's just the default. The DM still decides when you roll initiative.

[Malifice]

So in a recent game our bard managed to just waltz into an area we had to go through, convinced the people in the area that they worked for the same guy through some fast talking and faster spellcasting, and then got all the guards but one to run off. The one remaining thought that the bard was a childhood friend, and the bard talked the guard into a massage. During this, he literally stabbed her in the back. It was ruled at the table to be an instant crit, and the damage killed her, but it got me wondering about it and I couldn't seem to find any adequate explaination in the books for how to work this.

So, how would you handle a literal backstab to someone with their guard completely down and no reason to suspect or believe that anything might happen at your tables?

Mechanically:

Step 1: PC (or you the DM) declares hostile act.
Step 2: Roll initiative. This happens before the hostile act is resolved (more correctly this the first part of the resolution of that action). Creatures unaware of any hostile threats at the time initiative is rolled and hostilities break out, are surprised on round 1. They cannot take actions or move (barring a special ability that makes them immune to surprise like the Alert feat). After their turn 1 ends, they can take reactions.
Step 3: Creatures take turns in initiative order.

So in your case, when the Bard PC declares he 'stabs her in the back' both you (as DM) and the PC roll initiative. Your guard is surprised on round one and cannot take actions, or move on her turn. She also cant take reactions until after her turn ends.

Accordingly, the Bard PC gets to attack her first before she gets to act (although she may be able to take reactions to this attack depending on her initiative result). The Bard rolls higher in initiative to her he gets to stab her twice before she acts (on rounds 1 and 2).

If your Bard PC wants 'auto crits on surprise' he needs 3 levels in Rogue (Assasin).

Narratively

Is this some kind of mook that means nothing to the story? If so simply skip the above, describe the startled look on the guards face as she is stabbed, the fear and shock and tears as she realises she is going to die, the warm blood spilling over the Bards hands as his knife reaches her heart, her hands clutching his shirt as she slumps to the ground, and get on with the story.

[LeonBH]

There's no surprise round in 5e.

PHB 189. Surprise.

[...] In these situations, one side of the battle gains surprise over the other.

Taking from the OP's context, everyone was surprised.

You would get advantage, because the massaged creature propably lies prone.

He could have been sitting up while getting a back massage. The point is, being surprised does not grant advantage to the attackers.

[DKing9114]

This is one of those times where the combat mechanics and the reality they try to model come out of synch. DnD combat mechanics are intended to model a medieval-ish fight between more or less prepared characters, with surprise mechanics representing characters needing a few seconds to respond to an attack they weren't expecting. This is different-his target has dropped her guard, possibly even removed some of her armor, and he has plenty of time to set up the attack. I might allow/require one or both characters to make checks in the lead up to the attack (Insight/Perception for the guard to put things together, Intelligence to realize he can't be her childhood friend, Stealth for the Bard to draw his weapon silently, maybe even a Constitution check to see if the guard can get off a warning before she dies), but I don't know the full circumstances of what happened/what spells he used. However, when actually making the attack I'd be willing to give him an autocrit if/when the hit lands; for the same rationale that hitting a paralyzed or unconscious enemy causes an autocrit.

[Geeknamese]

I’d treat it as a surprise and I’d treat having someone lying facedown on a massage table, with no armor on, defenseless, completely trusting the situation as, for all intents and purposes, sleeping/incapacitated and open to an autocrit.

[Demonslayer666]

It would be a surprise round.

Everyone rolls initiative. Then, the first round is a surprise round. The attacker gets one turn in combat while the surprised parties cannot act, move, or react. After the 1st round is done, the 2nd round proceeds normally. There is technically no advantage granted to these attacks because the reward is already the surprise round.

The autocrit and advantage on an attack during a surprise round only applies to the Assassin Rogue. Handing it out carelessly nerfs any assassins in your party by making their 3rd level ability redundant.

Advantages are given out by the DM for whatever reason they see fit. This is a perfect example of when I would give advantage to an attack. Not only are they surprised, but they have been lulled into a false sense of security. They aren't walking along ready for a fight and get ambushed - that's being surprised.

[xanderh]

I'd personally just rule that the guard dies. Roll some ability checks to get into the situation of stabbing the guard like that, but once those are succeeded, the kill is earned in my opinion. No need to bother with the combat rules, the guard is without her weapons and armour, and probably isn't paying attention to what the bard is doing. Just enjoying the massage. At that point, there's no question as to whether the bard is going to be able to stab her fatally.
Maybe a check to see if the guard can yelp in pain loudly enough for someone else to notice, but realistically that's not likely to happen, considering she'll get stabbed in the heart or lungs, preventing any noise.