Glaaki in the mythos
From an oval body protruded countless thin, pointed spines of multicolored metal; at the more rounded end of the oval a circular, thick-lipped mouth formed the center of a spongy face, from which rose three yellow eyes on thin stalks. Around the underside of the body were many white pyramids, presumably used for locomotion. The diameter of the body must have been ten feet at its least wide . . . long stalks [were] twisting above it . . . [the] shape towered, pulsing and shaking with deafening vibration . . . a spine stiffened toward [a victim].
-Ramsey Campbell, "The Inhabitant of the Lake"
In Cthulhu Mythos fiction, 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 cult's holy book, known as The 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.