does black mask do anything when its charges are gone, or do you pay 500k for 10 charges and then its worthless?

It has the normal attack bonuses & defence bonuses anyways, The charges just release an attack once in a while~ :wink:

It still retains the slaying bonus even after the charges are exhausted.

Charged or uncharged, the black mask adds a constant 15% attk and str bonus during melee with Slayer Tasks only. It does not add any bonus to range or maging. And there is no bonus for any combat that is not a slayer task.