Campe

Cronus imprisoned the Hecatoncheires and the Cyclopes in Tartarus and appointed Campe as their jailer. She was a creature of spectacular ugliness — a woman from the waist up, a dragon from the waist down, with serpents growing from her legs, scorpion tails at her waist, and dark wings spanning the width of the prison corridor.

Nonnus gave the most detailed description: her body sprouted the heads of fifty beasts — lions, boars, dogs, bears — all growing from her waist like a living belt. Her hair was a nest of vipers. Her eyes flashed. She was, in every sense, a being assembled from nightmares.

When Zeus needed allies for the war against Cronus, he went to Tartarus to free the Hecatoncheires and Cyclopes. Campe stood in the way. Zeus killed her — the specific method varies by source, but the result was the same. The prisoners were freed, the Cyclopes forged Zeus's thunderbolt, and the Hecatoncheires provided the shock troops that turned the tide of the Titanomachy.

Campe was a threshold guardian in the purest sense. She existed to prevent something from happening, and the entire plot of the Titanomachy hinged on getting past her. Without her death, no thunderbolt. Without the thunderbolt, no Olympian victory. Without the victory, no Greek mythology as we know it.

She was also one of the few monsters killed directly by Zeus himself — most divine monster-slaying was delegated to heroes. That Zeus handled Campe personally indicated the stakes involved.