Summanus, Monarch of the Night, Lord of Hell, The Terror that Walketh in Darkness, appears as a grotesque, mouthless humanoid with pale tentacles sticking out from under his robes. He can use these tentacles to siphon blood from his victims.
Summanus had a following in Roman times, but if he is worshiped today, his cult is even more secretive. The rites needed for the proper worship of Summanus are found in the Tuscan Rituals.
Summanus is the creation of Brian Lumley — who based the Great Old One on the Roman deity of the same name — and first appeared in Lumley's short story "What Dark God?" (1975).