How do asymmetric PC exhaustion levels affect the difficulty of an encounter?

Question

As a DM, I have planned a medium encounter for my party of 5 characters.

Before this encounter, as a consequence of some non optimal choices, one of them suffers from 4 exhaustion levels, and there are no ways to remove one or more of them, because they can not take a long rest nor they have prepared a suitable spell such as Greater Restoration.

A total of 4 levels of exhaustion has quite an impact on the combat. The character has disadvantage on all rolls: this is equivalent to the effects of some spells (e.g., Eyebite), but in this latter case there might be chances to deny the spell's effects via a successful saving throw each turn. Moreover, the HPs of the character are halved, which means that they have (roughly) the HP of a character of half of the actual level\$^\dagger\$.

There are several indications about modifying the difficulty of an encounter in the DMG, under the Modifying Encounter Difficulty paragraph in the DMG (pag 84-85):

[...] Increase the difficulty of the encounter by one step (from easy to medium, for example) if the characters have a drawback that their enemies don't. [...] Situational drawbacks include the following:

• The whole party is surprised, and the enemy isn't.
• The enemy has cover, and the party doesn't.
• The characters are unable to see the enemy.
• The characters are taking damage every round from some environmental effect or magical source, and the enemy isn't.
• The characters are hanging from a rope, in the midst of scaling a sheer wall or cliff, stuck to the floor, or otherwise in a situation that greatly hinders their mobility or makes them sitting ducks.

None of the above examples seems to suit the case of exhaustion levels affecting just one character, but they regard situations in which the whole party is facing a severe drawback.

Is there any guidance about taking into account one character with exhaustion levels in computing the encounter difficulty?

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\$^\dagger\$ This may suggest to consider this particular character as having half of the actual class levels.

Answer 1

There are two cases here to consider. The first, a character like a fighter that relies on attacks for dealing damage. The second, a caster with access to spells which can force saving throws.

Case 1: Character cannot contribute by forcing saving throws

Take a good ole sword and board fighter. Their primary contribution to combat is being engaged in melee combat, making lots of attacks and taking some hits. 4 levels of exhaustion severely diminishes this character's ability to do anything in combat. They have disadvantage on all attacks, they are slow, and their hit points are halved. This character is unlikely to contribute much to the combat. If the character with exhaustion is someone like this, you might as well not count them in the party size when estimating encounter difficulty. They are likely to make things worse for the party as they try to keep this character alive.

Case 2: Character can contribute by forcing saving throws.

Now, suppose our exhausted character is a wizard or sorcerer who has access to a selection of spells that force saving throws. Their offensive capabilities are largely unhindered by 4 levels of exhaustion. Fireball doesn't care how tired you are, it still goes BANG! Sure, this character is even more squishy and vulnerable than before, but they are still in a position to contribute, even in an exhausted state, and they can do so while staying out of the fray. The fighter has to be in melee range to even have a chance of doing anything.

Conclusion: determine if the character can realistically contribute, if not, don't count them when considering difficulty.

There is obviously some in-between with these two cases, so you'll have to figure out where the character falls here. If they can still contribute in some meaningful way, you can just estimate that combat will be a little harder than expected. If the exhaustion severely hinders their ability to contribute, estimate difficulty by reducing the count of the party by 1.

If you're worried about a particular encounter being too hard, make it a two-stage encounter and adjust stage 2 as needed.

Every DM knows that encounter building is tough. Estimating encounter difficulty is far from being a well behaved science. When I am going into an encounter and I am concerned about possibly killing the whole party, I make the encounter a 2 (or more) stage encounter, so I can adjust later stages on the fly as needed. Essentially, I design an encounter that should be somewhat easy, and then I design a second part to that encounter with a range of possibilities. If part one is going great, I can drop the harder version of part two. If part one is going really bad, I can drop the easy version of part two, or get rid of it altogether. This allows you to get a real time sample of the party's performance so you can respond appropriately, hopefully resulting in an appropriately challenging encounter that doesn't kill everyone.