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I'm new to 5e, but not D&D.

I'm looking at the ability check and stealth mechanics and stealth looks really unreliable.

I'm picturing a group of 10 wood elves laying an ambush. Even at level 5, which is pretty good in the grand scheme of things, you only have proficiency +3. They're probably pretty nimble too with a dex mod of +3 as well. With only a +6 on a d20, at least one of them will consistently get a score below ten. This is setting an ambush when skilled and in their natural habitat, but it seems like they can't ever successfully ambush anyone, when I would think that in those circumstances they should almost never be seen unless the target is really exceptional.

Am I missing something? I figure I must be because otherwise it seems like nothing would ever get ambushed in D&D worlds.

What mechanics and strategies am I missing that make stealth work?

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You most likely have forgotten to apply modifiers to sneaking and hiding. Consider your example of Wood Elves in trees. The Elves should have proficiency bonus to stealth, and a bonus from dexterity as well, giving them roughly +6 to their stealth rolls.

Furthermore, they are hiding in trees, and hence are protected by foliage which either blocks sight completely (in which case they don't need to make a stealth check to be hidden if they are silent, as they are heavily obscured) or if the foliage is light enough to obscure but not block sight completely, it grants Disadvantage on active perception checks or a −5 to opponents' passive perception checks (see Hiding, Player's Handbook, page 177).

These two modifiers alone are enough to make a Wood Elf hiding in a tree completely invisible to the "Average Joe's" 10 passive perception, and very likely to beat even above average ones. Remember that a natural 1 is not an automatic fail for ability checks.

Another mistake you're making is assuming that all creatures of an ambush need to make a hiding check. Creatures that are completely obscured and silent (eg. behind walls) don't, and this is critical for the larger ambushes you mentioned. As long as there are enough hiding places or dark spots to completely conceal the bulk of the ambushers, it is enough for a few observers to remain in more visible spots where stealth checks are necessary to stay hidden. The rest can remain in positions where they don't necessary have a line-of-sight against their enemies, but can be alerted to spring into action by the observers.

In addition to what I pointed above, this could be considered a group check - at the GM's discretion. With a group check, at least half of the Wood Elves would need to succeed at their checks, which would significantly improve their chances of success.

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    \$\begingroup\$ Note that the elfs still have to roll Stealth if unseen (being unseen or heavily obscurred is a condition to even attempt the skill check). That is how you measure how silent they are. You can always give them advantage due to their preparation. \$\endgroup\$ – Nick Brown Mar 29 '18 at 14:36
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What mechanics and strategies am I missing that make stealth work?

This one (PHB p.175):

Group Checks

When a number of individuals are trying to accomplish something as a group, the DM might ask for a group ability check. In such a situation, the characters who are skilled at a particular task help cover those who aren't. 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.

Group checks don’t come up very often, and they’re most useful when all the characters succeed or fail as a group. For example, when adventurers are navigating a swamp, the DM might call for a group Wisdom (Survival) check to see if the characters can avoid the quicksand, sinkholes, and other natural hazards o f the environment. If at least half the group succeeds, the successful characters are able to guide their companions out of danger. Otherwise, the group stumbles into one of these hazards.

How can a group of elves help each other to set an ambush? Easy, they look at each other after they take up positions and point out "Hey, Glorfinwindle, I can see your ears sticking out from behind the tree."

In addition, if they are wearing appropriate camouflage then they probably deserve advantage. Conversely, if they are lightly obscured then anyone looking for them gets disadvantage.

In these circumstances a group that is as Perceptive as the elves' are Stealthy (+6) has a Passive Wisdom (Perception) of 16 - 5 for disadvantage giving DC for the elves of 12 (since they have to do better). For 10 elves with advantage a group check succeeds 72.92% of the time (http://anydice.com/program/ab73).

In addition, a partially successful ambush is still worthwhile - even if the PCs with high Wisdom (Perception) are not surprised, those with lower scores may be and would be unable to act on the first round of combat.

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  • \$\begingroup\$ This was a really great answer and I appreciate that you added it here. I don't think I can mark more than one post as the answer though. \$\endgroup\$ – MrGoodfix Feb 14 '17 at 4:39
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    \$\begingroup\$ @MrGoodfix I'd suggest marking this one the answer anyway. The first answer introduced some irrelevant to misleading material that completely overlooks the most important point of all. While a DM can certainly just choose to say 'stealth succeeds because they were silent', that doesn't provide players with the respect their choices deserve. A highly observant character with a great perception score is suddenly irrelevant because the DM just chose to give the enemy a success, just because. The group check answer better encapsulates why groups of stealthy foes will still tend to succeed. \$\endgroup\$ – RonLugge Feb 14 '17 at 23:47
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In this situation, if the party are the ones attempting to lay the ambush, I follow the Group Check rules on PHB p.175 (essentially having the more stealthy members of the group assist/make up for the less stealthy ones, pointing out better hiding spots/noisy footfall spots/etc). I would have the target DC be either the highest passive perception of the NPCs they're trying to ambush, or the value of an active perception check from the highest perception NPC depending on how alert they are. This reduces the impact of outlying rolls caused by a large party, but still requires the overall group to be fairly capable of stealth.

If the party are the ones being ambushed, I use their passive perception (if no one is actively looking out for an ambush) as a DC for a single stealth check made by the NPCs (or simply a target DC that you manually set, if you have a particular difficulty of spotting the ambush in mind). This is also the method used for the first ambush in Lost Mine of Phandelver.

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    \$\begingroup\$ I believe that if you are aware of any danger you cant be surprised. If your passive perception sees nobody than you are surprised, if you see even one enemy you are not. \$\endgroup\$ – Tijnkwan Feb 13 '17 at 23:57
  • \$\begingroup\$ @DaleM More useful than the closed+locked omnibus question, we have a specific question about that element of surprise: How to determine surprise when only part of a side is stealthy? \$\endgroup\$ – SevenSidedDie Feb 14 '17 at 1:04
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    \$\begingroup\$ I'm confused by this discussion in the comments about 'surprise' since neither the OP nor the answerer mention this mechanic. This answer boils down to 'use the Group Check rules', same conclusion as Dale M. \$\endgroup\$ – J. Foster Feb 14 '17 at 1:40
  • \$\begingroup\$ Well, you know the thing about surprise... it's ultimately where I would like to get to, but I know that a stealthy prerequisite to surprise is a successful stealth action. \$\endgroup\$ – MrGoodfix Feb 14 '17 at 4:07
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You should consider the fact that you can have partial success on laying an ambush. Unless the players are actively looking around, their passive perceptions are effectively setting a DC for the enemies to beat. There's a reason why the players send the Rogue ahead alone when scouting; there's fewer opportunities to screw up the stealth. If you have 10 elves, it's likely at least a few aren't going to be as well hidden as you think.

Upon entering the ambush point, some of the elves might have rolled below the threshold of the highest passive perception and others will have exceeded it.

So what this means is that a surprise round occurs (check the rules for surprise). And some of the opponents are aware of each other and unaware of others.

For example, a game I DMed this past weekend had the players encounter a trap laid by 2 hunter spiders. One spider's hide check was low and other was really high. The players were able to spot the low check, but that didn't negate the fact that another enemy was still hidden from them. Therefore, a surprise round occurred and the aware players could act, while the others could not.

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    \$\begingroup\$ This is not how surprise works - if the PCs detect a single threat they are not surprised by any threat. \$\endgroup\$ – Dale M Feb 14 '17 at 0:52
  • \$\begingroup\$ @DaleM More useful than the closed+locked omnibus question, we have a specific question about that element of surprise: How to determine surprise when only part of a side is stealthy? \$\endgroup\$ – SevenSidedDie Feb 14 '17 at 1:04
  • \$\begingroup\$ Thanks for weighing in. The DM's interpretation is certainly an important factor in the outcome. \$\endgroup\$ – MrGoodfix Feb 14 '17 at 4:41
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You have had plenty of good advice already. Key points:

  • Use a group Stealth check, or if they all have the same Stealth modifier, make a single roll for them.
  • Because the elves have had plenty of time to set up their ambush, give them advantage on their check.
  • Remember that obscurement imposes disadvantage on Perception checks.

Between them, these factors should allow some or all the elves to make decent Stealth checks, and some of, but not all, the PCs to be surprised.

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