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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.

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    \$\begingroup\$ As this is a major edit of the question, please don't vote to reopen this until OP has confirmed the changes have not changed the essence of their question. \$\endgroup\$ – Rubiksmoose Mar 21 at 19:23
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    \$\begingroup\$ Perhaps a good candidate to split off is the question (Q1) for other rule sources, other than PHB 189. Also, Q2 and Q3 can be stand alone questions. \$\endgroup\$ – svenema Mar 21 at 20:04
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    \$\begingroup\$ It is your question :) If you think the edit is too general, feel free to roll back and pare down/separate. \$\endgroup\$ – NautArch Mar 21 at 20:11
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    \$\begingroup\$ I'm also realizing that while my edit simplified for this particular situation - your questions may have been more general in nature for future use as well. If that's the case, I highly recommend submitting the general non-case specific rules clarifications as new questions. But a good answer here should also cover the issues related. \$\endgroup\$ – NautArch Mar 21 at 20:45
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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.

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  • \$\begingroup\$ "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" Alternatively give the others disadvantage which is -5. I always prefer to rule in favour of the PCs, \$\endgroup\$ – linksassin Mar 21 at 22:09
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    \$\begingroup\$ This answer is good. You could possibly improve it by mentioning that although the other party isn't surprised since they can see the non-stealth character, the stealthy characters are unseen. \$\endgroup\$ – linksassin Mar 21 at 22:11
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    \$\begingroup\$ @linksassin or simply make a perception check for the guard, who is actively looking out for intruders. After all, that's the whole point of checks ^^ \$\endgroup\$ – PixelMaster Mar 21 at 23:12
  • \$\begingroup\$ @PixelMaster I'd think the guard who's paying attention should have a better chance of success than the others. Getting an opposed roll instead of using passive Perception does not generally do that. \$\endgroup\$ – Mark Wells Mar 22 at 0:20
  • \$\begingroup\$ @MarkWells the guard's passive perception is still the minimum roll. If his PP is high enough to detect a stealthed creature, he wouldn't have to roll in the first place. If his PP isn't high enough, he can roll, since he's actively on the lookout. It's not gamebreaking to give the guard advantage here, but it's not at all how the game is """supposed""" to work. Furthermore, your PCs are gonna want advantage as well every time they are "actively guarding something", which is by default simply a situation where you roll for perception. \$\endgroup\$ – PixelMaster Mar 22 at 8:08
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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.


* 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.

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  • \$\begingroup\$ Note that a group check beats any DC up to the median of the group's results, so you can quickly determine who's surprised and who's not. \$\endgroup\$ – Mark Wells Mar 22 at 0:11
  • \$\begingroup\$ (This method has the problem that it's not any harder for a thousand people to sneak past a guard than for just one person. But using only mechanics specifically described in the published rules, it's probably the best we can do.) \$\endgroup\$ – Mark Wells Mar 22 at 0:16

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