Taking control of someone's commander when their entire decks depends on them having it can be a devastating blow, but most people build their decks to be resilient to commander hate. If it's a commander owned by another player, the player that looked at it turns it face up and puts it into the command zone. Roil Elemental Zendikar (R) 3/2 Cr - Elemental $12.99. Dedicated 1v1 edh decks are a VERY different thing.
(You and they can't be the targets of spells or abilities your opponents control.) (Put two +1/+1 counters on an Army you control. With the kami poised to overtake the mortals of Kamigawa, a black magician must set aside his own pursuits for the sake of the greater good The victory of the spirit world is at hand.
