Chthonians. (Aka Cthonians). The invaluable Cthulhu Mythos Encyclopaedia* has this to say about the Chthonians: 1) that they are a race of highly intelligent, very long-lived subterranean tunnel-diggers, 2) that they look like short-tentacled squids with no eyes, 3) that they are led by a seriously outsized member of their species named Shudde-M’ell, 4) that they are outstanding telepaths and 5) that they are very protective of their privacy and their young.
On the whole the Chthonians are inclined to leave others inhabitants of this our beautiful planet well alone as long as they, said additional dwellers, leave them alone in turn and don’t try and steal their treasured eggs. (Cave egg collectors. You have just been given wise counsel.)
They are practically invulnerable to almost anything you can throw at them bar high grade radiation and immersion in water. Also, a thingummybob called the Tikkoun Elixir, the Ankh, the Vach-Viraj chant -whatever that is when it’s at home- and the Elder Sign can disagree with them to some extent.
They are not technologically inclined, overall, although it is rumoured that some queer artefacts found in certain deepest parts of the ocean floor might have belonged to them.
Once every twenty-three years, the Great Old One Shudde-M’ell comes to the ruined city where the Chthonians once were imprisoned for a family reunion and to catch up with the latest gossip. This is always a grand, merry occasion, especially for the latest batches of offspring, who are, like children all over the cosmos, prone to mischief and partial to making very silly jokes and god-awful puns.
The Cthulhu Mythos Encyclopaedia, Daniel Harms. Elder Sign Press 2008