Grimask is the fourth boss in Black Castle (fourth assuming you did all the optional bosses). To reach him you must enter "Remuses Tower" and pass all the Black Castle challenges (See: Black Castle). Head straight forward and you will see a large library looking room. Kill all the adds because most likely they will pull when you fight him, and that is very bad.
He is a simple fight, sort of like a harder Tank and Spank. He does a bunch of AoE damage starting at his body and going outwards about 10 feet, so ranged stay back. Grimask himself does electric dmg so keep this in mind with Electricity Magic Protection spells. This AoE damage will most likely kill any non-tank melee's so pot often and multiple healers is a plus.