Taken from the RuneScape Knowlege Base: "Black Mask Black masks gathered from the corpses of cave horrors give a couple of bonuses when worn. Firstly, they provide Attack and Strength bonuses similar to a Salve Amulet when fighting your Slayer assignment. Secondly, the masks contain 10 spells that can reduce your opponent's Defence by 3-10 levels each time, regardless of whether they are your Slayer assignment, any other monster or another player. When the 10 spells are used, the mask and its natural bonuses will remain. " Note the last sentence.