This card might as well say, "as long as you control a creature, you cannot lose the game." 17
It's actually a promo in the Mega Charizard X ex UPC
I see the power here, but in a game thats been power crept since Coldsnap (?!??), a 6-drop that opponents have to decide they let trigger seems like a terrible deal
What this means is that, should the Troll take lethal damage from something (say from combat, or your opponent casting Pyroclasm), the Troll will stay alive as long as you pump two mana into him
Mirror of Fate can easily keep this under control, filling the library from exile whenever we need it
It also lets WotC cost cards with shield counters at a less prohibitive rate, allowing shield counters to appear on some pretty efficient creatures like Disciplined Duelist and Falco Spara, Pactweaver