Shudde M'ell
Shudde M'ell is a Great Old One created by Brian Lumley for the 1974 novel The Burrows Beneath. Shudde M'ell is an Ancient One of massive proportions with the fearsome ability to destroy whole regions with earthquakes. Shudde M'ell is the god of the Chthonians and they have the same form as he. These giant subterranean slug creatures can attain massive sizes, but nothing in comparison to the mile-long Shudde M'ell. While the Chthonians are unseen or unperceived by most humans living on the Earth, the Chthonian civilization is actually one of the most widespread of all the monster races. They are found everywhere throughout the world, though they live underground.
Shudde M'ell's Appearance
This is the description of Shudde M'ell in Brian Lumley's The Burrowers Beneath: "A great gray thing a mile long changing and exuding strange acids...charging through the depths of the Earth at a fantastic speed, in a dreadful fury...melting basaltic rocks like butter under a blowtorch."
While I'm not sure if phrases such as "like butter under a blowtorch" evoke the kind of horror you might expect from a Lovecraftian tale, you have to give Brian Lumley credit for being ambitious. While some other later authors who added to the Cthulhu Mythos tend to have lesser deities with more human or at least less alien traits, Brian Lumley creates cosmic monsters that are a mile long.
Furthermore, Shudde M'ell is described as the greatest of the Chthonians. While he's not worshipped much by cultists in the modern world of Lovecraft, evidence suggests that Stone Age tribes and druids once worshipped Shudde M'ell, which might explain its vaguely Celtic sounding name. The relative obscurity of Shudde M'ell is explained by his imprisonment beneath G'harne in western Africa, though the creature is now loose to wander the Earth.
Chthonians
Chthonians are a unique race in the Cthulhu Mythos. These elongated sacks of pulpy gray-black material live underground, but communicate with each other via telepathy. Adult Chthonians have a number of amazing abilities, including the ability to withstand 7000+ degree Fahrenheit temperatures. Chthonians also have the ability to burrow through solid rock (again "as if it were butter"), produce powerful earthquakes, and live everywhere worldwide, including in the basalt under the Earth's oceans. It should be noted that Chthonians are susceptible to water. While they can withstand small amounts of water, they would likely be destroyed if they were immersed in it. For this reason, they have powerful underground sonar abilities that let them the "low echo profiles" of watery sediment, allowing them to avoid underground aquifers.
Though they made their first appearance in The Burrowers Beneath in 1974, they were first mentioned in Brian Lumley's 1969 short story, "Cement Surroundings". These giant underground squids are said to be detectable by the high-pitched chanting which accompanies each Chthonian. The term "chthonian" comes from Greek mythology and is derived from the Greek word "chthon" (meaning "Earth"). The Chthonians were deities that were a dark, primordial counterpart to the sky-dwelling Olympians.
More Brian Lumley Lovecraft Horrors
Brian Lumley has added a number of deities to the Cthulhu Mythos pantheon. Creatures invented by Brian Lumley for the Lovecraftian cosmic horror universe include Great Old Ones like Mnomquah, Oorn, Summanus, and Yibb-Tstll, along with other monstrosities like Bugg-Shash, Kthanid, and Yad-Thaddag. The creatures Kthanid and Yad-Thaddag have drawn some derision from Cthulhu Mythos fans and critics, because they fall into the later habit of adding morality to the essentially amoral Lovecraftian cosmos. Both these creatures represent moral opposites to the cosmic deities Cthulhu and Yog-Sothoth.
For instance, Kthanid is described as Cthulhu's twin "brother", said to be as good as that dirty old Cthulhu is evil. Kthanid hates the other Old Ones because they murdered his family while he slept. Furthermore, Kthanid dwells in a cave in Elysia, which holds the Elysian Fields from Greek mythology. Yad-Thaddag, which appeared in Brian Lumley's novel Elysia (1989), is implied to be the counterpart to Yog-Sothoth--one of the mightiest of all Cthulhu Mythos creatures. Apparently, Yad-Thaddag is purely benevolent and looks almost like an exact copy of Yog-Sothoth, except its horrifying spheres are of a different color.
Shudde M'ell in Arkham Horror
Shudde M'ell is classified as an Ancient One in the Arkham Horror board game and he first appeared in the Dunwich Horror expansion. Shudde M'ell is a less powerful deity with only a combat rating of -3, though his doom track is a respectable 12. His worshippers, the Chthonians, deal more damage than most creatures, due to their innate destructive earthquake powers. They deal damage when they move on a roll of 2 through 6, instead of the usual 4 through 6.
When Shudde M'ell awakens, he's going to be more than the investigators can handle, though. Shudde M'ell has the "World Cracking" ability. When he stirs, you take out the 7 rubble tokens and have them ready for the next monster surge. Anytime a monster surge occurs, you must draw a rubble token and place it on the location it shows. This completely closes down the location for the rest of the game. Whenever there are no more rubble tokens to draw, the investigators lose. Also, whenever Shudde M'ell attacks, one undrawn rubble token is discarded. Whenever there's no more tokens to take away, Arkham is destroyed by a massive earthquake. This also means the investigators are devoured by Shudde M'ell and his Chthonian minions.