Void Worm

The Void Worm is a huge creature that resides deep within the Void Sea, underneath the Depths of Rain World. Its silvery body is indescribably long and gargantuan, almost like a massive snake or worm-like creature, and it has a small head with golden eyes and antennae-looking limbs connected to the edge of its long body. It only makes an appearance in the endings for all three difficulties. Although the player only encounters one void worm, many more can be seen swimming in the background.

Appearance (spoilers ahead)
In the Depths, when Slugcat jumps into the void fluid, swimming down long enough causes the Void Worm to appear. After a few moments of examination, it "hooks" Slugcat with a string, and then seemingly descends into the deepest parts of the Void Sea itself. This repeats a few times, and then it leaves. Upon being released, Slugcat is left in a pitch-black abyss with only a white light to follow.