Cthonian

"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

Cthonians, a.k.a. Chthonians, are a sapient alien species spawned by the Great Old One known as Shudde M'ell.

Biology
Cthonians resemble immense squids, with elongated worm-like bodies coated with slime. Despite their squid-like appearance, Cthonians are land-dwellers and are even harmed by water. They can feed on earth and soil, but prefer to consume the blood of animals.

Cthonians are oviparous and reproduce only rarely. They are powerful burrowers, live for more than a thousand years, and are highly protective of their young. It is said that a chanting sound accompanies every Cthonian, and that by such they can be detected while underground and unseen.

Cthonians are telepathically linked to each other. Their bodies are impervious to acid, highly resistant to explosives, capable of withstanding extremes of heat and pressure, and even remain undisturbed by laser beams. However, due to their alien constitution, they can't withstand exposure to water, which causes their bodies to corrode and dissipate. Additionally, they are also vulnerable to radiation.

Culture and society
An intelligent species, the Cthonians have developed the means to create artificial protoplasmic bodies that can house the disembodied brains of other creatures, keeping them alive long after their original bodies were destroyed.

Appearances

 * "Cement Surroundings", by Brian Lumley (1969)
 * The Burrowers Beneath, by Brian Lumley (1974)