Worms, as their name suggests, are tube like creatures. Unlike those found in nature, the worms of Eternal Darkness are quite large, and have sharp fangs and teeth. They appear mainly in the Forbidden City, where murals depict them receiving human sacrifices from an ancient civilization. Their appearances are rare and they never attack the player, but their presence causes a constant drain on the player character's sanity. They will retreat as soon as the player attacks them.
A worm is never seen in its entirety, so it is unknown if it is truly a worm like creature, or part of something far more massive. The tendrils that Mantorok uses to attack the guards who try to warn Ellia during her chapter appear quite similar to the worms.
A giant worm appears to lunge at the player in Ehn'gha, but this seems to be a hallucination as it instantly disappears a second later, after draining Edward's sanity. This seems to be brought on by a glowing purple pattern on the floor, again hinting at some connection to Mantorok.