During your Main Phase: You can change as many monsters your opponent controls as possible to face-down Defense Position, then you can send all face-up cards your opponent controls to the GY. If this card in its owner's possession is destroyed by an opponent's card: You can change as many monsters your opponent controls as possible to face-down Defense Position, then you can send all face-up cards your opponent controls to the GY. You can only use each effect of "Crystal God Tistina" once per turn.