Cerebus is the three headed guardian dog of the gates of Hades. The Guard Dogs of the Castle of the Crown would sometimes swear by Cerebus' collar.
Behind the scenes[]
In Greek mythology. Cerebus is the legendary guardian of gates to the underworld, Hades.
Alexander visits Land of the Dead, however, he never encounters Cerebus at the gates into the underworld. This suggests that that the beast is not the guardian of the realm of the dead. He may be a guardian of one of the afterlives which ghosts are moved onto after being judged by Samhain. As such Hades may be one of the next worlds.