The Malefic Mutation, is one of the two optional bosses beneath Black Castle which can be found by first taking a right or left down stairs from the entrance and entering the cellar. Once inside follow the path until you reach a tunnel in the wall. Follow this tunnel and take the left path into some Unholy Abberations. He appears as a massive ogre type beast.
Clear the entire room before fighting him, because he is constantly doing a bad Stamina/Mana drain spell, and fighting two mobs without any mana or stamina will be bad on your group. Don't hold back on potions during this fight, Remuses is the only other fight in Black Castle that you might need it more.