I like the general idea. You have 2 things here that I have always wanted: A slayer gem attached to your outfit (really thought this was going to happen with the jewelry update) and a Black Mask effect for Mages and Rangers. However I have quite a few problems with your specifics. First the levels are way too low. You max out at level 50, even though that is only 1/130 of the total XP needed to get to 99. I like a lot of your ideas, but you definitely need to raise the level requirements, otherwise too many of them are overpowered. Also another thing to consider is the value of the mask. Currently Black Masks alch for 2gp. Needless to say, when you die, you always lose your mask. These new masks are going to be untradable which usually have an alch value of 0gp. If I attach an onyx to something, I don't want to automatically lose that item if I die.