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Running Stealth Encounters

If a single PC is scouting out ahead, they should be making a stealth check, not having an entire encounter all to themselves. Encounters can easily take 20+ minutes to resolve. That’s not to say that a PC who decides to sneak alone should magically be able to resolve entire encounters with one die roll. Rather, lone stealth PCs should be scouting only a little bit ahead of the rest of the party, while solving entire encounters by just stealthing past them should generally be discouraged unless the entire party can participate.
A stealth encounter revolves around two basic types of enemy: Guards and sentries. A guard is someone who can plausibly kill a stealthed party (and especially a lone sneaker), whether by themselves or as part of a group that can easily alert one another. For example, a single orc may not pose a challenge to a level 5 party, but if thirty of them are all patrolling in one giant pack, they’re dangerous, and they’re guards. Guards are usually near-blind, with no Perception training and unexceptional Wisdom. PCs with Stealth training and who are at least level 5 have good odds of sneaking past a guard.
A sentry is someone who can plausibly detect a stealthed party. This means they need to have enough Perception to keep up with the Stealth of at least a Ranger (trained in Stealth, high DEX build), and sometimes even a Rogue (expertise in Stealth, high DEX build). A sentry isn’t necessarily strong enough to pose any kind of serious threat to the party members they detect, and in fact a pure sentry should be very weak, weak enough that a party can assassinate them without having a long drawn out fight. This is the Rogue’s forte, between their sneak attack and their Stealth expertise, and especially if they’re the Assassin archetype and auto-crit in addition. The Rogue is the most able to sneak past a sentry long enough to assassinate them so that the rest of the party can pass safely.
Making a sentry often requires adding the Keen Hearing and Smell trait to a low level monster (monsters who already have this trait make good sentries automatically). This gives the monster advantage on Perception checks related to hearing or smell, and a +5 to their passive Perception score. This gives them a big leg up to help balance out what is often a middling WIS and a relatively low proficiency bonus compared to the PCs, who are higher level (since sentries are weaker in combat). For example, adding Perception training and Keen Hearing and Smell to a goblin turns a passive Perception of -1 into a +6, high enough to make them a threat to stealth-focused Rangers as high as level 5. A Ranger of that level would have +4 DEX and +3 proficiency, which is a +7, barely higher. This ranger would have no difficulty assassinating one goblin, but runs a serious risk of being detected by a goblin sentry. Thus, the Ranger’s problem is how to kill the sentry without being detected by it, and without alerting the guards. It may make more sense for certain sentries to have something like Keen Sight instead, or an entirely original trait. Maybe your Bearded Devil sentries have learned a special technique that allows them to detect the souls of Material Plane creatures and this gives them advantage on detecting them.
Combining the guard and the sentry gives us the sentinel. A sentinel is almost like a wall, with the high perception of a sentry and the killing power of a guard (and like guards, that killing power might come just from having a bunch of them in one spot). They are too tough to be assassinated before alerting reinforcements, and too perceptive to be safely snuck past reliably. Sentinels need to be avoided, tricked, or distracted. Adding Perception training to any high WIS monster of a similar CR to the current party will make them a Sentinel. For example, an orc ranger might take regular orc stats, but add Perception training and higher WIS and DEX, with lower STR to compensate, making him a good level 1 sentinel. If their WIS is low and it would not make sense to increase it, a sentry trait like Keen Hearing and Smell can be added instead. Not every stealth encounter needs a sentinel, and often when seeing a sentinel in the enemy party a sneaking party will either abandon stealth or leave the encounter and try to find an alternate route.
When a sentinel also has good stealth capabilities and tends to remain hidden, they are a predator. Usually they are not only strong enough to survive the first round of combat and alert nearby guards, but strong enough to serve as a mini-boss fight for the party on their own. Predators are usually encountered alone (they can be used as wilderness encounters), but they can also work alongside guards.
Finally, there is the officer. The officer thwarts a different approach to stealth than has been discussed so far – social stealth. That might sound like this is in the wrong section, but disguising and deceiving your way past guards has much more in common with sneaking past them than with negotiating their defection. The officer is a sentry but with Insight instead of Perception (and he can be combined with combat power or stealth abilities to make officer sentinels or officer predators).
All or nothing stealth means that failure is basically inevitable. Six players roll Stealth, one of them will roll lower than passive Perception in almost any encounter even against monsters with a passive Perception of only 10 or 11, even if all of them are Stealth trained and ready to be a part of a stealth-focused party. Failing a Stealth check does not result in immediate detection. Instead, every monster whose passive Perception beat the Stealth roll goes up one level on the following spectrum.
Unsuspecting: “Gods, this job is boring. I hope something exciting happens soon, I don’t care what it is.” An unsuspecting creature counts as surprised and attack rolls against them have advantage. Creatures, even when on guard duty, are almost always at this level by default. When the party attempts to sneak into an area containing an unsuspecting creature or attempts to take a significant action (such as stealing something off a wall or opening up a prison cell or similar) they must make a Stealth roll to avoid detection by an unsuspecting creature. If the Stealth roll fails, the unsuspecting creatures become suspicious.
Suspicious: “What was that noise?” Suspicious creatures count as surprised and attack rolls against them have advantage. If they were previously unsuspecting, they immediately make an active Perception check to try and locate whoever it is that just made them suspicious. If the Perception check fails, they become unsuspecting again. When creatures are expecting an imminent break-in, they may be suspicious by default, but sentries (and guards and etc.) cannot be suspicious all the time. If creatures are given reason to be suspicious, such as by finding a dead body or being informed that there are intruders loose in the building, then they will become suspicious and remain suspicious for the next several hours. Suspicious creatures, in addition to immediate Perception checks if they became suspicious as a result of a failed Stealth check, will provoke Stealth checks against their passive Perception under the same circumstances as unsuspecting creatures. Failing any Stealth check regarding a suspicious creature will alert that creature.
Alerted: “I saw something! Someone’s in here!” Alerted creatures do not count as surprised and attack rolls against them do not have advantage (unless they have advantage for some other reason). They immediately shout a warning that makes all other creatures in earshot suspicious (unless the alerted creature is themself trying to hide, in which case they will not reveal themselves) and make an active Perception check to try and locate the creature that alerted them. Any creatures made suspicious by the alerted creature will also make Perception checks. If an alerted creature succeeds on their Perception check to find the creature who alerted them, they become aware.
Aware: “You there, halt!” An aware creature has located the sneaker. They can make attacks like normal and will shout the location to any nearby creatures. These creatures are alerted and immediately make a Perception check to find the sneaker. Any alerted creatures who succeed on their Perception checks become aware, and then initiative is rolled. Any creatures who are merely alerted do not get to act in the first round, and if all aware creatures are killed in the first round of combat, the sneakers can attempt a Stealth check to avoid detection (unless they’ve done something that would cause them to be immediately detected, like making loud noise or running out in the open). Any creature who is attacked in melee is automatically detected. Generally speaking, once one sentry or guard becomes aware, the stealth encounter is over and a combat encounter is beginning. However, it is not necessarily every sneaker who’s been detected, so other party members may be able to attack from hiding spots or even ignore the fight completely and continue sneaking.
No encounter should be designed in a vacuum, but never is this more true than when approaching from the perspective of the stealth encounter. Sentries (and sentinels and predators) aren’t standard monsters, they have special training and sometimes even non-standard traits in order to keep up with Ranger or possibly Rogue stealth checks. They’re unusual and rare, which means the bad guys don’t generally have a very large number of them, so they’ll usually be either posted at important chokepoints or else sent to patrol through multiple areas, because the baddies simply do not have enough to put a sentry in every single room (even though they probably can do that with guards). A stealth encounter is more about finding a way past the chokepoint while dodging the patrols, which means designing the opposition is as much about what other opposition is nearby as it is about what opposition the players are dealing with right now.