I am in the process of designing an encounter in which a party of 8 level 3 players will fight a group of Chuul.
The CR calculation shows that 3 Chuul will cause a hard encounter, but I intend that the battle have an underwater element.
How does fighting underwater increase the encounter difficulty of a battle?
I will be applying the underwater combat rules as written.
party is made up of
Barbarian
Artificer
Warlock
Sorceror
Fighter (who mainly uses his bow)
Ranger (who mainly uses his bow)
Rogue (who mainly uses a bow)
Cleric
The idea is that the Chuul are in a pool of water so will swim down and force the players to come to them.
Best Answer
It might increase the difficulty of the encounter one step.
There is some guidance in the encounter design section of the Dungeon Master’s Guide that is relevant here:
In your scenario, if none of the party have a swim speed, the encounter taking place under water would definitely count as a drawback here for the purposes of estimating encounter difficulty. The Chuul, having a swim speed will find underwater combat to be as easy as combat out of water, your party however, will be at a disadvantage:
On the other hand, if the party would be mostly unimpeded by fighting underwater (by everyone having a swimming speed), fighting underwater would not affect the encounter difficulty at all.