Chthonians (/ˈθoʊniənz/; from Greek: chthon, "earth") are massive burrowing worm-like creatures. The species is the creation of Brian Lumley and was first featured in his short story "Cement Surroundings" (1969)—though the creature never made a direct appearance. The chthonians had a more prominent role in Lumley's novel The Burrowers Beneath (1974), whose title was taken from one of the stories said to have been written by Robert Blake in Lovecraft's "The Haunter of the Dark."
Flowing tentacles and pulpy gray-black, elongated sack of a body...no distinguishing features at all other than the reaching, groping tentacles. Or was there—yes—a lump in the upper body of the thing...a container of sorts for the brain, basal ganglia, or whichever diseased organ governed this horror's loathsome life!—Brian Lumley, The Burrowers Beneath
Chthonians are described as immense squids, with elongated worm-like bodies coated with slime. Despite their squid-like appearance, chthonians are land-dwellers and are even harmed by water. Chthonians are powerful burrowers, live for more than a thousand years, and are protective of their young. It is said that a chanting sound accompanies every chthonian, and that by such they can be detected while underground and unseen.
The most important individual chthonian is the gigantic Shudde M'ell, which is worshipped by the rest.
In Greek mythology, "chthonian" refers to beings that inhabit the underworld; they are considered to be the dark, shadowy counterparts of the Olympians.