Oh, I think I found it:
"Kerberos is a mythical dog with either three or fifty heads, depending on which ancient Greek author you consult. His function in Greek religion and literature was to be the Hound of Hell, the monster guarding the entrance to Hades, the underworld of the dead, simultaneously keeping the dead from returning to the land of the living and preventing the living from visiting the region of the dead (as tourists) prematurely. "