A Beautiful Knife

Take control of a creature an opponent controls.
Untap it, and it gains Haste until end of turn. If
that creature deals combat damage to an
opponent this turn, you may sacrifice it to deal
damage equal to it's power to all creatures that opponent controls instead.
"It's too lovely a night to be alone, good Ser. Would
you like some company? I know a perfect place
we can enjoy the knife."