Surprise Attack within Party

[Maelynn]

If the Rogue attacked during an encounter when the players were engaged with other enemies, how would you handle it? Any check at all? Surprise at all? Is the attack done as part of the normal initiative/combat moves?

Okay, putting on my DM hat. This is just my opinion and how I would rule, it's my interpretation and might not be strictly RAW.

No initiative roll required, as the party is already in a combat encounter. Follow normal initiative order. If the Rogue wants to do a surprise attack, then he needs to move in behind the victim and roll Stealth against their Passive Perception. If he succeeds, then he gets a surprise attack against the victim - this doesn't mean he gets advantage, so no Sneak Attack damage. It does mean that the victim gets no reaction, because they haven't had a turn before the Rogue reveals his intention (the reaction during a surprise round is that split-second you see something happening and can react if you're fast enough)... so the victim doesn't have their turn between the moment of reveal and the Rogue's turn (since they're at the same time), so they get no reaction.

I'd also rule that party members who have a clear line of sight at the victim's back (as in, standing behind them and not next to or in front of them) get a check as well, because they can clearly see the Rogue moving in. If their Passive Perception is higher than his Stealth roll, then they're allowed to use their reaction against him if they have an action that allows it (like Sentinel feat, or Rally maneuvre).

Oh, and about the poison: you can easily borrow a blade and then coat it with a poison from a vial you carry. Just saying. :3