How can I fit infiltration-style encounters or quests into the combat framework of D&D 5e?

Question

I’m running a 5e campaign, and our group is going into a period of the campaign that will likely involve a lot of ‘sneak in sneak out’ encounters/series of encounters, like assassination missions or thievery.

The problem is, I’m not sure how to keep it balanced with the so-called ‘adventuring day’; it seems like if they plan it well, or even if they don’t, they should have at most two fights in a row, and those in very quick succession, because they can hardly sneak in, have two fights in a row, rest for an hour (!!) without being caught, and then have two more.

How can I fit infiltration-style encounters or quests into the combat framework of D&D 5e?

(NB: I’m afraid the system is more or less non-negotiable; there’s a pandemic, and even if there wasn’t, we’re a good few levels into the campaign; it just wouldn’t work to change up systems.)

As requested in the comments, the party consists of two rogues and a Circle of the Moon druid, all at 3rd level.

Answer 1

A combat is just a way of draining resources

While most of the rulebook concentrates on how many combats you can manage in a single day the reality is that each combat is simply a way that the game gets the player characters to expend a certain amount of resource.

The idea is that at the end of each day they have used up those resources, and need to rest to regain them before they can move on.

You can do exactly this with other encounters.

Sneaking into a place can be as simple as 'make a Dexterity (Stealth) check to slip past the guard' which uses no resources and reduces infiltration to a matter of selecting the correct skill at character creation. I find that boring, and leads to what you worry about which is that a good roll leads to success and a poor roll leads to being caught potentially more dangerous than they would otherwise be capable of dealing with.

What I do is try and fill the path they take with other encounters which are aimed at draining resources, and with methods of achieving rest.

Encounters

For example: "You spend some time watching the patrol routes of the guards and realise that it simply isn't possible to sneak in using the rooms and corridors. However you spot a series of grates which you believe lead to an underfloor heating system, if you could get down there the grates in each room could potentially allow you to infiltrate almost anywhere in the building"

The players thus have to go under the building to sneak in, and you can fill that with traps, obstacles, gelatinous cubes or whatever else you think appropriate intended to use the same resources as a few combats.

The end result is they get to the same place within the building, but it took resources and skill rather than a single good roll, and they are less likely to get caught by every guard in the building.

You can achieve the same result by ensuring they reach a junction in the building and have to go left to avoid the guards, this will put them into a non-combat resource draining encounter.

Resting

If the group are under the building they can rest as they see fit, but once they go into the house that is a different story. However now they know there are ways to get under the house, maybe distracting the enemies, quickly killing them or filling the room with fog will allow them to simply disappear and the guards will be left wondering where they went. Failure could lead to the guards spotting and following, but you are in control of this so could simply make the players worry about being followed, but actually you have no intention of allowing it (if you run games in that way).

Alternatively; the guards would just want to capture them, and you could plan for them to be locked up for a while which gives them the chance to have a short rest and them look to escape. Which is always an adventure in itself.

TL/DR

Don't think of combat as the only thing that the party can encounter in a day, you can drain resource in other ways.

Don't let people potentially achieve a goal with just a good roll.

Answer 2

The game isn't just about combat, and infiltration/assassination aren't just about avoiding it

It mostly is, by rulebook focus, and the idea of the "adventuring day" is a part of that-- it is defined by how many resources the party is expected to have expended to make sure that they feel challenged. Depending on your group's preferences, you can use other types of challenge to make them feel similarly. But in general, you should not feel like you've made a mistake just because the party had less than 6-8 combat encounters in an adventuring day. It's also the case that not every day needs to exhibit the same extremity of challenge; even if you like the suggested encounter rate per day, not every day needs to be so challenging.

Further, there are a lot of ways you can work explicit challenges into these types of quest, including combat encounters:

Restrict rests

This is the tactic I find most useful. Rests are a substantial part of why the adventuring day features as many challenges as it does. The party gets to expend resources in fights, and then gets some resources back through resting. It's easily in your control, as DM, to make resting impossible or highly undesirable on a given quest. This is equivalent to getting a full adventuring day's worth of challenge out of fewer encounters without changing encounter balance. The system isn't all that tight (I can't give you a formula like one fewer short rest = a full adventuring day equivalent at 5 encounters), and it will hit different PCs and classes differently, so it will take some flexibility and experience to make such adjustments well.

Fights as a consequence for failures

A good plan and good rolls may well let the party slip past guards and traps, but poor plans and unlucky rolls will not. They can try to be as sneaky as they want, but they are unlikely to succeed all the time, and you get to decide how that plays out. A single alert might make further sneaking effectively impossible, or might double guard patrols, or something similar. Plan A is to get in and out undetected, but the party had better have a plan B!

Fights on the way in, fights on the way out

Depending on what they're infiltrating, they may not be able to just walk up to the target area and start sneaking. If they have to cross a dangerous, monster-filled moat to get to the castle, you can prod them into a fight before they can get to the sneaking. Maybe the assassination target has a kennel of dire wolves released to find the target's killer after the body is found but before the PCs have escaped. The general idea is that the sneaking can be a subset or special phase of the overall quest in the adventuring day.

Snowballing problems

If the party gets into a fight, the players will probably win it. Say they do so without raising an alarm. Great! But now they've got a pile of corpses, a bloodstained room, and bits of evidence of their passing (clothing scraps, etc.) all over the place. The party now has more problems to deal with, and those problems can drain resources themselves or simply make further stealth much more difficult. During a sneaky infiltration, there is no reason to think that a fight solves more problems than it creates.

External, limiting factors

If the party has a time limit, and it turns out that the only way they can accomplish what they need to before it's up is to be less sneaky and more reckless, they have to choose what's more important to them. It may be more fights and better rewards, or more stealth and fewer rewards.

PC-enhanced sneaking is a thing, and can allow for combats as well

It's dependent on the party's composition and the choices players have made, but spells like Silence can allow fighting and sneaking at the same time, subject to some risk of the latter. If being undetected is important, but fights are unavoidable, then the game's challenge becomes more than just the fights-- it's the fights along with an extra, challenging objective.

More vicious traps

As above, encounters in an adventuring day are meant to drain PCs' limited resources in service of providing an overall degree of challenge. You can simulate that very easily, without compromising the sneakiness of the quests, with traps and the like. I very much prefer traps to combat for this sort of thing because they're more predictable: PCs may or may not succeed on a save, but the damage is going to be in a predictable range.

Misdirection as stealth

Being unseen and unheard isn't the only way to be stealthy or to pull off a job like theft or assassination. It might be just as effective, and maybe desirable, to be seen while wearing an NPC faction's uniforms. Maybe the goal is to stage an assassination attempt that "fails" and is noticed, to prompt an NPC to reallocate resources or inflame some pre-existing suspicion. Quests should not all be the same, and if the central element of several quests in a row is "sneak in and out, invisibly" there may be a broader issue with how they're designed and set into the campaign.

Remember goals and constraints

In the same way you probably would not design a dungeon in which the major obstacles are enemies, and create a plan to allow PCs to casually skip those obstacles, you do not need to plan infiltration missions that have no obstacles. It's totally fair for perfect stealth to not be achievable (unless the quest explicitly requires that), or challenges which might enable a stealthier approach requiring hard-to-roll successes or precise in-game knowledge. You can also introduce challenges directly relating to the sneaking, though for that to feel the same resource pressure you might need to homebrew some mechanics to flesh out those aspects.

Legendary specialists are... legendary specialists

Ultimately, if your players' PCs are focused on this kind of stealth they might be (or become) very good at it. If that's the case, and you don't want to homebrew mechanics to make infiltration itself richer and more challenging, it may be wise to have these missions be shorter, less detailed, and more prone to success. If a player knows their character will be going to Ice Town, and takes a bunch of fire spells in response, it would be unreasonable to nerf that PC just because they're specced precisely for the challenge and therefore get an easier adventuring day. Don't let the tail wag the dog: the point of D&D is not to deal with 6-8 combat encounters per adventuring day. If that encounter rate serves your purposes, great, do it! If not, then do something different.

Answer 3

Make harder encounters.

If players get to pick their battles and rest between encounters without consequence, you can adjust the difficulty of encounters. By making things harder each fight, you maintain the challenge for players and emphasize the importance of sneaking around and cleverly picking their battles.

I wouldn't do this for all encounters, though. It is important that players are rewarded for their clever tactics by steamrolling through what would otherwise be regular encounters. But maybe after a few easy fights, the enemies have noticed that half the army seems to have vanished, so now they patrol in groups and frequently.

Give players places to rest

Use secret rooms hidden in officer's quarters where players can hide bodies and rest before continuing. Have other enemies come in the quarters and PCs overhear a conversation to show how the remainder of the dungeon is being affected.

Where is Boss Man? He should be here. Luke and his patrol vanished from the front gate, and Boss Man is nowhere to be found either! What is happening?!

Answer 4

The General Case

So, most answers here already cover the general ideas, but I will go over them as well.

The idea of the adventuring day as stated is to try and fairly balance the different classes, some which depend on short rests, some which depend on long rests. As noted in this question, changing the adventuring day to a shorter version, which is what you are planning to do, changes the balance of the game in favor of classes with bursty, recover-on-long-rest resources. A Wizard now may use all of its spells against the enemies in the first two encounters, because these are all the encounters.

If you try to balance that, for example, by making the encounters significantly stronger, that will make the encounter problematic for classes that don't have that many resources to begin with, because they are supposed to recover them at their short rest.

This creates a huge imbalance within the classes, making some classes greatly stronger than others, which is usually perceived as a problem.

I do not know any way of solving this issue in a general setting. If you remove short rests, you are making classes that depend on short rests weaker, just like if you put a giant anti-magic field in the campaign you make classes that depend on magic weaker.

However, in your case, the lack of short rests is not due to an actually shorter adventuring day, rather due to the (lack of) plausibility of taking a rest after the enemies have been warned.

In that case, instead of depleting the resources through combat (i.e., enemies that actually see the PCs and would call for other enemies), you can slowly take away their resources while not making them perceived by the enemy forces yet, thus, allowing for a short rest.

So, this is the first general advice:

Deplete their resources and Do allow Short Rests

In this sense, SeriousBri's answer is probably better than what I can advice you, but note that it is generally hard to take away resources such as Action Surges, Spell Slots and others with traps and other non-combat stuff. The only way I found that worked in my groups was through puzzles that required some specific resource to be spent. I have used a "Damage check door" which required the characters to deal some target amount of damage in a single round in order to open it (forcing them to spend resources as it was virtually impossible to deal that amount of damage without spending some stuff), runes that would only activate if someone cast a Fireball on them, and other ways to consume resources (that I obviously knew they had because I designed them after knowing what the party was :P).

In your case, it is quite hard to do thematically, since they are invading an enemy fortress, and having that fortress to have something "tailored" for the characters to overcome sounds awkward. But maybe one of the main enemies is a beast shapeshifter, and he has a locked vault that can only be opened by... a Bear! - essentially forcing your Druid to spend one of his transformations on it. This idea is a little far fetched and the example is quite silly, I admit, but I hope it is good enough for you to see the idea - and I am sure you are able to think on something along these lines that makes sense in your setting and that your players will appreciate (personally I would love to be the Druid and have a challenge that has to be overcome by me transforming in some animal).

Very important note: If you follow this idea, make sure you give enough clues for your characters so they understand what must be done and not waste more resources than you expected. In the Fireball example, it would be quite bad if they first tried Burning Hands, Scorching Ray and Chromatic Orb - Fire before actually trying a Fireball - that would lead to way more resources spent than I planned and would make the following encounters nearly impossible.

Your specific party case

The thing is: Your party consists of two rogues and a moon druid. Rogues are, in fact, remarkable for being almost a resourceless class. The only resource they care about is HP, essentially. They do not get spell slots (not the Thief or the Assassin at least), extra consumable dice, or any kind of feature recovered on-reset. That makes it considerably easier for you to balance out.

The Druid, as well, only has one relevant short rest resource: their beast transformations. With a one-hour-long transformation, that should not be a problem in terms of resource, either, and most likely they are going to be transformed for most of the combats at this level in any adventure.

So, from my point of view, the only resource you should worry about is their HP. This is the easiest resource to take a little bit away.

Make it a little bit harder

Unless your enemies already deal high damage, you can slightly increase the damage of the enemies - not enough to make it a whole CR higher, but enough to be noticeable in that they may leave the two-encounters-in-a-row in a very bad shape, maybe even dropping one of the Rogues' HP to zero (make no mistake: he should not die).

Alternatively, you can set traps - traps that deal damage are, well, the easiest and most common ones, compared to traps that force a spell slot to be spent - so they arrive at the encounters missing a few HP.

As a final alternative, I have found out that giving surprise to the enemies is a huge advantage for them and will make the combats considerably harder, without effectively increasing any damage dice, modifier or HP of the monsters/NPCs. Instead of rushing towards your party when they notice the invasion, they may handle it a little bit better and wait for a moment where they will have the upper hand. The guards will not immediately ask for help, though, as they want the glory of defeating the invaders for themselves, and will only ask for help when they notice the invaders are stronger than they initially thought. (Surely surprising your party is not that easy, but it is a possibility for balancing it).

Give them ways of recovering resources and make them go through a "normal" adventuring day

What is more interesting about the particular party setup you have is that you can give them ways of recovering their most important resource - HP - that are not short rests. Give the Druid a Cure Wounds scroll. Give them a few Potions of Healing. Then you can actually make them go through a few more encounters, and almost look like a "normal" adventuring day, but without short rests between the encounters. Give them enough potions and 1 minute to actually drink them and your party can even go through 4-6 encounters in a row, which is something most parties would have a problem even if they had a significant number of potions, because they have other important resources, like Ki Points or Action Surges or Warlock Spell Slots.

Ultimately, if, due to the nature of your party (two rogues and a druid), they can effectively invade in a very stealthy way and go through only one encounter, for example, these extra resources (potions and scrolls) you gave them become a reward for handling the challenge in a more efficient way, otherwise, they just spent what they were supposed to spend and no harms done.

Answer 5

If there is a place your party wants to infiltrate, then this place should be serious business. A place like this wouldn't have 2 stupid guards napping at the door and one poor lock to break to get into.

It would have multiple security instances. Glyph of Wardings, Magic Mouths, Alarms, maybe even anti-magic fields, multiple guards patrolling in unforeseeable patterns, placing fake "loot" (whatever they are searching there) and hiding the real one even better.

Combining all of this with the answer of SeriousBri, the combat in the basement with the cube or something, you can drain their resources pretty efficiently. This will make the evaluate every step they take and lets them use their brains to handle this instead of their dice alone.

Every time my party sneakily opens a door I roll a "luck check" to see if this door gives a loud squeak and see if the PC can react fast enough. This way even the highest slight of hand check can lead to exciting situations.