In this scenario, how do I determine whether the enemies are surprised?

Question

Being new to D&D and planning to do some DM'ing in the near future, I found myself struggling with "Surprise" in combat. To educate myself on this topic, I first want to understand the surprise mechanic rules as written.

Let us consider the following scenario:

4 PCs are attempting to sneak up on 3 monsters who are behind a hedge

PCs have rolled stealth rolls of (21, 13, 9). The last PC is not attempting to be stealthy at all.

Monster passive perceptions are (18,11,15). Only the 15 is actively on guard duty, but the other two are awake and aware.

In the above scenario, how do I determine if the PCs are successfully stealthy and if the surprise mechanic is used?

Good answers will include consideration of stealth, passive perception, whether or not a creature can be surprised at all, the differences between group vs individual stealth, and anything else I may have missed.

Answer 1

The rules say:

If neither side tries to be stealthy, they automatically notice each other.

from which I conclude that any character that is not trying to be stealthy is automatically noticed. So all of the monsters have noticed your "not trying to be stealthy" character.

The rules say:

Any character or monster that doesn’t notice a threat is surprised at the start of the encounter.

and all of the monsters have noticed your "not trying to be stealthy" character, so none of them are surprised.

However, some of the monsters are not aware of your other characters. Those characters might make their first attack with advantage, if they attack a monster that wasn't aware of them.

You've noted that one of the monsters is "actively on guard duty" while the others are merely "awake and aware". The DM might choose to give the monster that is "actively on guard duty" advantage on its passive perception check, which adds +5 to its passive perception. This is at the DM's discretion.

If your group wanted to try harder to be stealthy, the DM might allow some of them to stay "farther behind", making them harder to notice. For example if they stay 100 feet away, the DM might give them advantage on their stealth roll, or even waive the stealth roll entirely. This is at the DM's discretion.

Answer 2

Since several PCs are attempting the task as a group, the Group Check rule applies. From PHB Ch 7. - Group checks:

When a number of individuals are trying to accomplish something as a group, the DM might ask for a group ability check... To make a group ability check, everyone in the group makes the ability check. If at least half the group succeeds, the whole group succeeds. Otherwise, the group fails.

The group check mentions an ability check. What would be the DC? First, M3 is on guard duty, an average intelligence creature might receive advantage on it's passive score.*

I'll assume M3 is our average Orc or Goblin, and is just on guard. For the group check, I see three separate level of success:

• DC 11 for surprising M2
• DC 18 for surprigint M1 and M2
• DC 20 for surprising all monsters

With the given rolls of 21, 13, 9, half the group (21 and 13) beat the DC 11.

If the PCs are sneaking up the monsters from cover or hiding, you may use the same stealth check for hiding. I would ask for a new check when the PCs change hiding spots.

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* If the creature has a supernatural ability of being super aware of it's surroundings, the DM may decided he cannot be surprised at all.