Bel-Shamharoth
Bel-Shamharoth (also known as the "Soul-Eater," the "Soul-Render," or the "Sender of Eight") is an ancient, dark god for whom the description "evil" is inappropriate; "negative" and "void" might be better terms. Bel-Shamharoth is all suckers, tentacles, mandibles, and one giant eye. His dilapidated temple is built of two-to-the-power-three-sided stones, with 7a sided tiles, lit in an eerie dark violet and possibly octarine light by twice-four-sided crystals, and a number of main corridors one more than seven and one less than nine leads to the center of the temple, where there is a slab with the same number of sides as a spider has legs. Rincewind, unwillingly visiting the temple, has likened it to a spider's web. In addition to leading the visitors to the center, whichever way they may turn, the temple also is bigger on the inside than the outside, a quality often found in buildings that do not occupy real space-time. The temple and Bel-Shamharoth himself appear in The Colour of Magic. The temple is long since abandoned, worship of the Sender of Eight being a decidedly short term prospect. These days he is mostly remembered in the name of the Young Men's Reformed Cultists of the Ichor God Bel-Shamharoth Association (which is also known as the Young Men's Pagan Association or YMPA). His likeness is etched on the cover of the Octavo.
It has been speculated that Bel-Shamharoth is a Creature from the Dungeon Dimensions who has managed to cling onto the Discworld and gathered worshippers (see The Discworld Companion). This might mean he is in fact less cruel than creatures from the Dungeon Dimensions (whose want for physical existence is what causes them to be cruel), just more capable of hurting non-wizards and therefore quicker to gain a reputation.
Bel-Shamharoth could definitely be seen as a version of H P Lovecraft's Cthulhu.
In Pyramids it seems he is still remembered and referred to by the people of the Djel as the Eater of Souls.
See also: Numerology, Wizard's magic