Byatis, the Serpent-Bearded, is a fictional deity from Ramsey Campbell's 1961 short story "The Room in the Castle". Byatis was a Great Old One, his idol brought down to Earth by the Deep Ones. Some time later, under unknown circumstances, he was sealed behind a stone door in what would later be the Severn Valley . Most likely the Elder Gods were responsible. Upon the Roman conquest of Britain, a small group of soldiers formed a cult to the god, which was annihilated upon his short-lived escape. Centuries later, a Norman castle was built near what would become Berkeley and Severnford . During the 1700s, Byatis was placed under the control of the warlock Edward Morley , who trapped the monstrous creature in the dungeon of the castle, using its powers to enhance his own. He fed it by letting it loose during the night to devour the townsfolk, him in close succession, then returning to the castle and locking it away once more. However, with each living thing it ate, the bigger it got. Eventually it became too big for its prison, finally trapping itself by consuming its keeper. This and the memory of the Great Old One's earlier escapes developed into the legend of the Berkeley Toad.

Byatis appears as a spidery, crab-like entity with many legs and clawed forelimbs, a mouth wreathed in tenticles, a long elephantine proboscis, and a single, large, supposedly hypnotic eye. Its size is known to be incredibly large, one of its face tentacles as thick around as a man, making it about the size of the castle it lies beneath to this day. He is able to be summoned by a living being touching his image and is repelled by the Elder Sign. He is mentioned in De Vermiis Mysteriis.