Gla'aki 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). Gla'aki has the shape of an enormous slug covered with metallic spines that, despite their appearance, are actually organic growths. Gla'aki 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, Gla'aki was freed, and the impact created the lake wherein he now resides. Gla'aki 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 Gla'aki, was written by his cult, which gleaned sorcerous knowledge from their master. While the original text was reputedly written in eleven loose-leaf notebooks by various unidentified contributors, the 1865 edition published by the Matterhorn Press of Highgate is contained in nine volumes, described as "edited, organised and corrected". A small edition printed exclusively for subscribers, it is now extremely rare.