This is OP. You can fly it and buff it, and that way you'd go on killing anything... I mean, at least make it low atk.
Instant-kill can be too powerful by itself, but then you add it to a good atk to cost ratio weapon... It's not balanced.
I say deal it's attack as damage to a creature and lose 1 atk or something like that, and anyways, you should low the atack a bit, since it's good as a weapon by itself, but you're adding a cool effect to it.