Let’s come up with a monster.

I don't remember that (I'm not a big fan of TES).

These are Necromancers, worm-like humanoids inhabiting the underwater kingdom of Thras. They were responsible for the Trassian Plague, which occurred in the Second Era and claimed the lives of half the inhabitants of Tamriel. They used to live on the mainland, but after the plague, the Imperial troops almost completely destroyed all the Sloads and sent their kingdoms to the seabed. Sloads are immobile and can move using teleportation.
Mentioned in The Elder Scrolls Adventure: Redguard. The game features the great necromancer N'Gasta.