Stealth is going to be a thing in Undermountain. Well, I guess, I think it will be a thing. I mean you guys (my players) can all go fecklessly blundering into whatever encounters you like. Don’t let me stop you.
But assuming it’s a thing, this explains the rules as I see them and how I’m going to apply them within the Frauds of Waterdeep campaign.
The important thing to bear in mind is that stealth is also an opposed check; the target DC to spot is the outcome of the Stealth check rather than static.
Stealthing as a Group
There will be occasions where you as a group want to move silently and hide in shadows (oh, 2e I love you so!). This is what happens:
1. Everybody makes a Dexterity (Stealth) check. This is going to set the DC of Perception (checks or Passive) to know you’re there. You make this check exactly the same as normal, factoring in any bonuses or advantage etc as usual.
2. We work out the mean average of the Stealth check results. Example, if your individual rolls were 8, 9, 12, 20, 28 then the average is (8+9+12+20+28) / 5 = 15.4, rounded down to 15. Therefore the DC of a Passive perception or Wisdom (Perception) check to spot the group, or anyone in the group is 15. (Of course individuals can separate off; we’ll adjudge this at the table.)
That’s it. The hardest part is the maths, but how hard can it be right?
Stealth House Rules / Clarifications
Note these are general rules; specifics of the situation may over-ride these.
Stationary Creatures are Harder to spot (House Rule)
It just kinda makes sense that if you’re not moving around then you become harder to spot. As such, I’m going to grant advantage to Stealth checks for stationary creatures (i.e. a bonus to the hide roll; the spot roll remains unchanged).
Half or more cover makes it harder to spot (House Rule)
Similar to the rule above, it just makes sense that if only my head is popping up it is harder for you to spot me. As such, I’m going to grant advantage to stealth checks for creatures with cover (i.e. a bonus to the hide roll; the spot roll remains unchanged).
Invisibility and Silence (House Rule and Clarification)
Invisible creatures cannot be seen, but can give themselves away by lots of other factors. However, it makes sense that it’s harder to spot them so Invisible creatures (as per the condition) get advantage on Stealth rolls as well as the ability to Hide in Plain Sight.
Silent creatures (e.g. in a zone of a silence spell) also get advantage on Stealth rolls, however, they can’t Hide in Plain Sight. I know that this is probably unfairly balanced towards silence but its an easy way to deal with it and it also feels appropriate in the likely scenario in Undermountain (sneaking around having not been seen already).
Invisible creatures should still make Stealth rolls if they want to be properly hidden.
Invisible creatures retain the benefits of the invisible condition even if their Stealth roll is lower than the observer’s perception. The observer knows that they’re there, but not exactly where. So spells that rely on sight are foiled, attacks against them are at disadvantage, their attacks are at advantage etc.
Intelligent creatures may, however, raise the alarm etc as they know an invisible creature is around and about.
Pass Without Trace (Clarification)
This grants a flat +10 to the Stealth check only if you remain within 30′ of the caster for the duration. This is as per a tweet from either Mike Mearls or Jeremy Crawford clarifying this.
Light and Sound in Undermountain
Darkvision allows you to see in normal darkness as if it was dim light. Dim Light is the equivalent of lightly obscured. Perception checks in lightly obscured situations suffer disadvantage (or -5 to Passive checks).
So all of the group reliant upon Darkvision make Perception checks with disadvantage (as do your opponents with darkvision). Iggy’s Devil’s Sight ignores this.
Magical Darkness and What Cancels it
Magical items that emanate light do not count as “magical light” for the purpose of cancelling magical darkness. My rationale is that, whilst how they create the light may be magical, the light itself is not inherently magical.
It is worth noting that darkvision does not penetrate magical darkness, and this applies as much to your enemies as it does to you. Being within magical darkness imposes the Blind condition, as it creates a heavily obscured area. Also worth noting that Iggy’s Devil’s Sight can see into magical darkness.
I’ve put together a table of how far sounds like combat, fireballs etc will travel in Undermountain to help me referee this kind of thing. Areas of tight corridors, or thick wooden doors between adjacent rooms will muffle sounds. Areas of open rooms, or long straight corridors may well amplify sound.
So if you were fighting at the far end of a room adjacent to a populated mess hall with a closed door then sounds of combat may not penetrate the background noise of the mess hall. However, if the door is open it may well do so.
I’ve decided that fireball is more like a whoomph! of a bonfire being lit, than the Crack! of a bomb going off.I’ll rule on other spells as we come across them, but if you know the spell you can ask what it sounds like.
I’m not planning on sharing the details of the table, but you can start to infer it from your experience if you like.