I've been a 5e player for the last 6 months or so and soon I'll be DMing for the first time, as short filler when our main campaign can't go ahead (for example, our paladin is on holidays).
My campaign centers around a town terrorized by an Aboleth, which the heroes have been contracted to deal with. Aboleths are smart, so unless the heroes do something interesting, it will likely try to fight in the water where it will have the advantage.
I have read through the DMG and some other online guides but there doesn't seem to be a consistent way to run this style of battle.
I understand that movement speed should be ¼ of the heroes normal movement, unless they've got some modifiers or such.
The main things I need to understand are:
- How does spell casting work underwater?
- How should breathing mechanics work underwater?
Best Answer
Firstly, you didn't ask, but movement is effectively cut in half, rather than quartered. Per the rules on climbing, swimming, and crawling:
So movement is ⅓ if the heroes are in difficult terrain - normal swimming is only ½.
Next, spellcasting. There are no specific rules about underwater spellcasting, but the description of verbal components says:
It's pretty reasonable to extend this to a character underwater, so spells with verbal components are out. There doesn't seem to be anything else that would restrict underwater spellcasting, however.
Finally, breathing mechanics. The rules for suffocating are on page 183 of the PHB, and are much simpler: