Two of my other favorite Old Ones are Y'golonac & Glaaki
Y'golonac (the Defiler) is a fictional deity in the Cthulhu Mythos. He is the creation of Ramsey Campbell and first appeared in his short story "Cold Print" (1969).
Y'golonac is a Great Old One and is the god of perversion and depravity – not just "average" human perversions or depravities, but any that can be conceived of by a sentient being (sane or otherwise). His demeanor is much like that of Nyarlathotep, but he is much more perverse and sadistic. Y'golonac can sometimes be summoned merely by reading his name (not necessarily aloud; just reading it is sufficient) in the Revelations of Glaaki.
Beyond a gulf in the subterranean night a passage leads to a wall of massive bricks, and beyond the wall rises Y'golonac to be served by the tattered eyeless figures of the dark. Long has he slept beyond the wall, and those which crawl over the bricks scuttle across his body never knowing it to be Y'golonac; but when his name is spoken or read he comes forth to be worshipped or to feed and take on the shape and soul of those he feeds upon. For those who read of evil and search for its form within their minds call forth evil, and so may Y'golonac return to walk among men . . .
—Revelations of Glaaki, Volume 12[1]
Y'golonac is imprisoned behind a wall of bricks in unknown ruins. His true form is uncertain, but when he possesses a human host to manifest, he appears as a grotesquely obese man, lacking a head or neck, with a mouth in the palm of each hand.
Unlike most Lovecraftian deities, Y'golonac is obviously capable of understanding humans, to the point of being able to conduct a conversation in English through his human host. Y'golonac seeks humans who read perverse and forbidden literature to become his servants. When Y'golonac is summoned, he offers to grant the summoner the dubious honor of becoming his priest, or simply kills him for food.
Glaaki
Glaaki is a Great Old One and dwells in a lake in the Severn Valley near Brichester in England (though he has been reported in other lakes around the world). Glaaki has the shape of an enormous slug covered with metallic spines that, despite their appearance, are actually organic growths. Glaaki can also extrude tentacles with eyes at the tips, allowing him to peer from underneath the water. It is believed that he came to the Earth imprisoned inside a meteor. When the meteor landed, Glaaki was freed, and the impact created the lake wherein he now resides.
Glaaki is an ancient and wise creature with vast knowledge of the other beings that are active in Britain's Severn River Valley, such as Y'golonac, the Denizens of S'glhuo, Shub-Niggurath, Eihort, and Byatis. The twelve-volume Revelations of Glaaki was written by his cult, which gleaned sorcerous knowledge from their master.
Glaaki's cult (Servants of Glaaki)
By driving one of his spines into a victim and injecting a special fluid, Glaaki can turn the unfortunate into an undead slave; however, if the spine is broken off before the fluid is injected, the victim dies anyway but is at least spared the fate of becoming Glaaki's slave. The injected fluid produces growths throughout the victim's body that allow Glaaki to manipulate the subject's corpse.
Many people come to serve Glaaki willingly, in exchange for the promise of eternal life. What they don't realise is that he makes good on his promise by driving his spines into them, turning them into undead slaves.
The Green Decay
As time passes the undead creatures become increasingly sensitive to sunlight, and even begin to take damage from it. The Servants of Glaaki refer to this condition as the Green Decay.