Giratina, the Renegade

Giratina, the Renegade is not controlled by
any player.
First Strike, Trample, Vigilance
At the beginning of combat on each player's turn,
that player may sacrifice a creature. If he/she
doesn't, Giratina, the Renegade attacks that player. Giratina, the Renegade can only be blocked by
one creature.