Shudde M'ell is the ruler of the Cthonians, and is said to be the largest and most powerful of that race. Little is known directly about this being, though it can be assumed that it travels around the lower strata of the Earth, spawning more progeny and keeping itself alive.
It is pictured as a large burrowing worm with mass of tentacles on its front end. At least one author describes it as a mile long, and that may not be an exaggeration.
There are no known cults for Shudde M'ell, though there are some small groups that try to contact Cthonians for the hidden knowledge of the Earth that they possess. One would suspect that Sudde M'ell would be well respected in those circles.