Cerberus is a dog-like monster that dwells in the entrance of the underground passageway to Canifis. He can be safespotted with Ranged and Magic. Cerberus is weak to fire spells.
You are required to kill Cerberus in the Priest in Peril quest. It can only be killed once, after which it is replaced by another dog named Lycurgus. Previously, trying to attack it after the quest gave you the message "I'd better not to make the King mad at me again!".