Imo it's a bit much. This would be absurdly strong on rejuv steals specifically. It also would make a single teammate being melee hungry result in losing entire teamfights in close quarters. (And thus his entire team flaming him afterwards)
It's a neat idea, but feels like it would just become a low elo clipfarming item, with it being virtually ignored above oracle. Rn I think parry is in a god position, in that unless you're building melee items people just aren't meleeing lategame, and if they are they're usually going for secured melees (Think abrams stun, billy on a whiffed parry, etc.)
I don't think you can balance an AOE stun off parry, and would probably feel un-fun to be on the receiving end of.