I'm wondering if a short vuln up would be more beneficial instead of a -10% mitigation, save this kind of thing for an actual party utility skill. You said it yourself, its an act of retaliation. Maybe instead of an outgoing damage debuff we get some kind of attack power buff to go with our next bloodspiller which can be increased even more with dark arts. There's lots of options but damn, I sure would like to have that animation/sound combo as a skill regardless.