The Gravecreepers standard attacks are a ranged attack, where he throws a spinning axe, and a melee attack, where he swings the sword he holds. The ranged attack can drain your prayer points. The melee attack is extremely accurate and fast while the ranged is semi-accurate and tends to hit higher. When the boss re-emerges the room will be partly filled with purple liquid, and he is likely to be standing in the middle of it.