Rogue sneak attack and what does it mean to be an enemy? [duplicate]

Question

The Rogue can sneak attack if another enemy of the target is within 5 feet of it. What constitutes being an enemy - Hostile intent or hostile actions or just being a passive ally?

1. An ally of the rogue has the Sanctuary spell up and is within 5 feet of the enemy. The Sanctuary spell continues as long as the warded ally does not attack or casts an attack spell (and maintains concentration). Assuming the warded ally does not attack, would it be considered an enemy for enabling the rogue's sneak attack?
2. An ally of the rogue has Gaseous Form active and is within 5 feet of the enemy. Gaseous Form does not allow the ally to attack. Does this enable sneak attack?
3. What if the ally is a warlock with Cloak of Flies active and damaging the enemy?

Answer 1

"Enemy" and "hostile" are not game terms, they are plain English. I think most people have a good enough understanding of that term to agree on the vast majority of situations. When they can't, it's up to the DM to decide if the sneak-attacked creature would consider the other creature to be their enemy.

In a battle where the party has come across some goblins, and the rogue is trying to sneak attack a goblin, it's fair to say that in all of those cases being an "ally of the rogue" is enough for the character to be an enemy of the goblins.

It doesn't matter if the character can attack or not, them existing is enough.

Answer 2

This is up to the DM, but probably means "someone distracting"

The description of sneak attack declares certain requirements to be met:

another enemy of the target is within 5 feet of it, that enemy isn't incapacitated, and you don't have disadvantage on the attack roll

The things are clear about distance, disadvantage and incapacitated condition, so it seems rules as written a creature can trigger sneak attack regardless of anything. Since the only requirements are to be within 5 feet and not to be incapacitated, a quiet invisible spellcaster hidden from the enemy still gives an opportunity for sneak attack. Doesn't it?

Well, let's read the beginning of the sneak attack description:

Beginning at 1st level, you know how to strike subtly and exploit a foe's distraction.

Since there are no ignorable "fluff" in 5e rules, we should take this text into consideration as well. Sneak attack is the classic "back stab" trope — the foe faces away for a moment and the rogue exploits this. The rogue does not just magically deal damage, (s)he "strikes subtly" and "exploits a foe's distraction" instead. If there is no distraction, there is nothing to exploit.

So an "enemy of the target" requirement presumably means the narrative truth about the target being paying attention to that creature. At the very least the target has to be aware of that creature and has to consider it an enemy of itself. Of course there are corner cases, but I don't think this is really a problem — when in doubt, a DM can always ask the Rogue player "how do you do that".

Answer 3

The other answers do a good job of addressing general cases, but since neither of them give specifics on the cases you ask for, here are some answers to your specific questions:

An ally of the rogue has the Sanctuary spell up and is within 5 feet of the enemy. The Sanctuary spell continues as long as the warded ally does not attack or casts an attack spell (and maintains concentration). Assuming the warded ally does not attack, would it be considered an enemy for enabling the rogue's sneak attack?

The warded ally could always change their mind and attack, breaking the spell. It is perfectly reasonable to think this situation qualifies as an "ally of the rogue" that is in some way distracting the enemy. They could choose to take an attack of opportunity should the enemy run, for example. It'd break Sanctuary, yes, but the enemy can't know whether the ally will choose to attack or not. I don't see anything RAW that prevents this; neither do I see a consistent interpretation of the rules that prevents this.

An ally of the rogue has Gaseous Form active and is within 5 feet of the enemy. Gaseous Form does not allow the ally to attack. Does this enable sneak attack?

This one is a bit trickier; since the target of the spell cannot take attacks of opportunity, I could see DMs ruling either way. However, do consider that the spellcaster could stop concentrating on the spell, thus allowing the target of the spell to attack again. The spell doesn't seem to prevent the target of the spell from aiding an attack, so I would say this falls into the same precedent as a familiar counting as an ally for sneak attack (see Crazjtk's comment, also this discussion on Reddit: https://www.reddit.com/r/dndnext/comments/49h83w/regarding_sneak_attack_feature_can_a_familiar_be/). So it seems that per RAW this works for sneak attack; this certainly would meet the minimum threshold of distracted by an ally of the rogue. If that cloud next to you could materialize at any moment into a wizard, I think you'd be distracted too.

What if the ally is a warlock with Cloak of Flies active and damaging the enemy?

Most definitely yes; the warlock is within 5ft of the enemy at this point, and (assuming that the warlock is a fellow PC or other ally of the rogue) is no different than having any other ally next to the enemy. Cloak of Flies doesn't prevent you from attacking, or enemies from seeing you. Even if the warlock didn't have weapons, the warlock could still punch the enemy. Punches hurt too.