That's actually not bad. I would keep the "flagellate/vampire" theme and make it so each enemy you hit with the first cleave gives you a buff stack, and instead of a flat %dmg increase which is very dangerous with scaling, i'd make that your next X (= buff stacks) abilities deal %more damage as shadow and consume a stack. If a single target is hit, you get 5(?) stacks.
More istant damage, less buff/debuff management (that we have already).
EDIT: our aoe is capped, but stuff like Blade Rush is not. The initial damage cleave could be uncapped aswell, but i think we should get a capped amount of stacks.
EDIT2: it's fun how everytime players are "called" to give their own ideas, most of the times they come up with stuff that's more interesting and actually viable without being clunky or not doable in game (since there is already other stuff that work in similar ways).