Can an NPC rogue (who pretended to be allied) use Sneak Attack against a character, even without advantage or another hostile creature nearby?

Question

Our DM had an NPC who essentially "joined our party", but was actually an enemy rogue.

In the middle of combat, the NPC ran up and attacked a party member (i.e. a player character); the DM deemed it a surprise attack, and allowed the NPC to get Sneak Attack. (No other creature was within 5 feet of either the NPC or the party member; it was just the two of them.)
The goal was basically to one-shot a low-level character while the rest of the party was unaware.

Is it normal to allow a rogue to get Sneak Attack in a situation like this, where the attacker isn't hidden and otherwise doesn't have advantage?

I know in the end it's pretty much up to the DM's discretion, but I can't find where that fits in the rules, and it almost resulted in a one-hit KO.

Answer 1

This doesn't work, unless the DM wants it to work.

The conditions for triggering sneak attack damage are quite clearly spelled out:

Once per turn, you can deal an extra 1d6 damage to one creature you hit with an attack if you have advantage on the attack roll. The attack must use a finesse or a ranged weapon.

You don’t need advantage on the attack roll if 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 situation you describe does not sound like it lines up with the conditions. Nevertheless...

The DM can decide you have advantage.

The rules do not explain every possible circumstance that may grant advantage or disadvantage. To account for this, the DM is given discretion here. From the rules for advantage and disadvantage:

The DM can also decide that circumstances influence a roll in one direction or the other and grant advantage or impose disadvantage as a result.

At their discretion, the DM may decide that the circumstances warrant giving a particular roll advantage or disadvantage. It sounds to me like the situation you describe is definitely surprising, so advantage on the attack does not seem unreasonable, and would then trigger sneak attack damage.

This ruling is kind of lame because of misaligned expectations.

The reason you are asking this question is because you understand how sneak attack normally works, and the situation where sneak attack was used against your party member did not line up with that understanding. Your expectations and what happened did not line up. To be clear, this is not always bad. Interesting plot points that subvert our expectations are good. Subversion of expectations that lead to player characters dying are pretty cheap (in my opinion).

Most people don't go into the game expecting to cheap shotted by the DM, and this sounds like that is exactly what happened.

Moving forward, it may be best to discuss this move with your DM. This answer has some guidance for getting that ball rolling, and this article has some more in depth guidance to think about: What kind of game do you want to play?

Answer 2

As a ruling, it makes sense

In 5e the DM awards advantage when someone tries to do something in favorable circumstances, and disadvantage when circumstances are unfavorable: Advantage and Disadvantage

The DM can also decide that circumstances influence a roll in one direction or the other and grant advantage or impose disadvantage as a result.

If your friend runs up behind you and stabs you, that would be quite unexpected - you would probably allow your friend to get a lot closer to you than you would allow a hostile goblin to. It is definitely reasonable to say that the attacker has advantage in this situation.

Attacking a target when you have advantage and are using either a finesse or ranged weapon is enough to trigger Sneak Attack:

Once per turn, you can deal an extra 1d6 damage to one creature you hit with an attack if you have advantage on the attack roll. The attack must use a finesse or a ranged weapon.

Every DM has different opinions

Every game is different, so it's hard to judge what "normal" is. However, this ruling is sane and appears to make sense. I would rule this way, and I would guess that a lot of other DMs do too.