Underworld Cerberus

Feared by the living and the dead, cerberi patrol both banks of The Tartyx River. These multiheaded hounds of The Underworld breathe gouts of molten rock that sear and imprison those who trespass upon the borders of life and death. Most cerberi have a boundless hunger for fresh meat, especially the flesh of Humanoids. Villains have been known to exploit that hunger by luring cerberi away from the river and setting them loose on mortal Settlements.
Three-headed cerberi dwell deep in The Underworld. Terrifying and cunning, they guard Portals between the wards of The Underworld, Erebos's greatest Treasures, and noteworthy souls who might attempt to Escape the realm of the dead. Three-headed cerberi commonly serve Demons, but if left to their own devices, they often herd wily souls into labyrinthine Underworld wildernesses, then hunt them for sport.