It's mana cost for it's ability is extremely powerful. I would suggest giving it flash, since it's not only WU, but it would make it more versatileand a staple. It also may not have to be mythic. It could just be rare. Overall, I give it an 8/8.
According to another card that actually explains it its a keyword that reads: "Whenever this creature deals combat damage, target opponent looses that much life." (or something with the exact same affect.)