It certainly is OP, but I'm a sucker for references like that, so. Anyway, with that in mind, the nerf I have in mind through it is that it doesn't just nullify the damage for the person who has it; they still suffer the damage, and just end up returning it afterwards, worse. I'm rusty on my Bible verse, but I'm pretty sure the Mark of Cain involved that whatever happened to Cain would return sevenfold. So, because of that, in my mind the first thing that needs to be changed about it is that whatever damage is coming have to be suffered before it can be reflected. It fits the reference more, and also gives it a significant nerf, because if whoever has it gets killed well, yeah, their attacker dies, but sucks to suck they're still dead. Does that make sense?