You must have at least 10 defence AND 20 strength AND 40 combat to equip a black mask. Once you wear a black mask it raises your strength and attack by 15% during slayer tasks. A black mask has (10) charges which lowers your opponents defence. It will activate randomly. Once your 10 charges are up you do not have to buy another one, as the 15% still works at (0). A black mask will work in conjunction with prayer bonuses. Black masks require 40 combat to wear.