What Would Live A Under water under dark?

Ask the tittle says my settings equivalent to the underdark is a network of lightless, submerged caverns that stretch across the world. However I can't think of any creatures to live there! (These include thinking beings) Strach that there is one I can think of.

The Drowned. The Drowned are a race of blind aquatic undead made from those that drown. They hunt based on sound and vibrations in the water. Their society is a Theocracy that worships and offers the dead to The Void Born aquatic godlike horrors from before life.

So any Ideas would be nice.

How about some equivalent to angler fish? Say, its maw and "lamp" looks like it does on an angler fish, but rather than fins, it has sticky legs with which it attaches itself to the ceiling or walls of the caves and sticks there, waiting for its prey.

For things it could prey on, how about some sort of small pack creatures? I'm thinking about something that looks like a pale axolotol (like here◊), only rather than legs, it had fins and it swims around in packs. It could feed on some sort of coral/moss-thingy growing on cave walls.

Deep Ones.

How about Mermaids who are part human, part cave fish.

Combining a human like face with the cave fishes Eyeless Face would be creepy sentient being and a real head screw for characters. "Mermaids you say? that sound lov, OH MY GOD WHERE ARE THE EYES!!"

In D&D, several monster races had aquatic equivalents:

Koalinth = marine hobgoblin

Merrow = aquatic ogre

Scrag = sea troll

Kapoacinth = sea gargoyle

Lacedon = sea ghoul

Post two and four have the right idea I want unique interesting aquatic creatures.

The descendants of pirates who thought to use the caves as hiding places and got hopelessly lost. Eventually, they devolved into cave-dwelling monsters. Imagine an eyeless humanoid with transparent skin and oversized ears.