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?