The party goes to sleep around the campfire, and one member volunteers to take the first watch. A couple of hours later, a dozen bullywugs try to stealthily ambush them.
The DM makes stealth rolls for each of the attackers and compares them to the passive perception of the character keeping watch. Inevitably, one of the bullywug rolls fails, and thus the ambush is detected. (Group stealth is nearly impossible, it seems.)
So the watch character is not surprised, and initiative ensues. But the character rolls a low initiative.
What happens next?
Do the bullywugs get to attack the sleeping characters before the watch can wake them? (At advantage, automatic crits, due to unconscious condition.)
That's how I played it as DM, but it seemed a little harsh. Am I parsing the rules correctly?
Random encounter during a long rest, with a watch, is a very common situation. What is the correct way to resolve it?
Best Answer
The DM controls the narrative, not the mechanics
The "correct" way depends on the table expectations and the genre you are playing, not the game mechanics. To an extent, the DM can make sleeping in the wilderness as dangerous as she wants to. D&D 5e puts aside the mechanical approach to stealth, leaving it up to the DM almost completely. You can find some 'Sage Advice' on stealth in this podcast.
As a DM, think about decisions that the players could make and the consequences they should get as a result:
The next paragraph is analysing the ambush from the players' perspective.
What was the point of the watch if it didn't prevent an ambush?
The very point of staying on watch is to be able to wake up the party before the danger bursts in. If the only advantage of the watch is the chance of the watcher (not) being Surprised, the watch itself becomes meaningless.
It's a good idea to give a chance of spotting the enemy beforehand. However, rolling for every bullywug and comparing with the passive score is the antipattern known as "rolling to failure". The rules suggest using a Group check instead.
It still requires a lot of checks for NPCs, which isn't considered a good practice. The game is a story about player characters, not about NPCs.
There is a better alternative.
Ask for a Wisdom (Perception) check from the player and compare it with the passive Dexterity (Stealth) score of the bullywugs. If he succeeds, let him spot the enemy far enough to be prepared. If he fails, the danger bursts in - all the teammates are asleep and unconscious. The watcher being (not) Surprised makes little difference in this case.