Void Worm

Void Worms are huge creatures that reside deep within the Void Sea, underneath the Depths of Rain World. Their silvery bodies are massively long and gargantuan and they have small heads with golden eyes and antennae-like limbs connected to the end of their long bodies. They can be encountered in the endings for all three difficulties. Although the player only encounters one void worm that actively interacts with it, there are many others that can be seen and swum past. Slugcat is forcefully moved in the same direction as a void worm when its body passes directly overhead, accompanied by a loud rumbling sound.

Encounter
In the Depths, when the slugcat jumps into the void fluid, swimming down long enough causes a void worm to appear with a unique rumbling sound. After a few moments of examination, it "hooks" Slugcat with a string, and then pulls slugcat upwards, then back to the depths of the Void Sea before releasing Slugcat and vanishing. Upon being released, Slugcat is left in a pitch-black abyss with only a white light and music to follow to the end of the game.

Trivia

 * There is a possibility that void worms are related to the Leviathan, although the only suggestion of this is the body type and swimming motions.
 * In versions of the game earlier than 1.5 it was possible to be killed via fall damage from an invisible Void Worm in the room before the void sea.