This rare, reviled monster can loosen its cellular structure to pass through organic materials like some intangible spirit, enmeshing its body with a living host and seizing control of its mind. As it absorbs mental energy and nutrients, the host is stricken mad and slowly deteriorates into a crumbling, mummified husk. The Endo will perish with its host, but leaves behind a clutch of black, hairy eggs.

Endo have entirely alien thought processes, but seem to feel an abstract form of love for their hosts. In battle, an Endo will attempt to send its snakelike tongue into the body of an opponent and damage vital organs. It produces rapid clicking sounds when pleased.