Endworm

The Endworm is a titanic, monstrous worm-like creature, and is considered the largest and most destructive of all the inhabitants of Europa.

Biology
Endworms, named for their ability to "end" any life they come across, are by far the largest of all the creatures found on their homeworld, easily dwarfing the largest known Earth whales. Their anatomy makes them resemble bristle-worms, a type of Earth marine worm. The body has a dark rusty color.

Behavior
Solitary, Endworms have no natural predators apart from each other, as they are top predators of their range. As their mandibles aren't too coordinated to capture small prey, they only hunt larger prey items, which includes creatures the size of a submarine.

Many reports of missing expeditions and crews had been blamed on the Endworm for its attacks on submarines, forcing crew members to evacuate into the ocean and lose contact.