Whenever Gruul Ragebeast or another creature enters the battlefield under your control, that creature fights target creature an opponent controls.
1/24/2013 Gruul Ragebeast’s triggered ability isn’t optional. However, if no opponent controls a creature when a creature enters the battlefield under your control, the ability will be removed from the stack.
1/24/2013 If one or both of the creatures that are supposed to fight are no longer on the battlefield when the ability resolves, the fight won’t happen. Neither creature will deal or be dealt damage.
1/24/2013 If the target creature is an illegal target when the ability tries to resolve, it won’t resolve and none of its effects will happen. Neither creature will deal or be dealt damage.