People are blowing this off, but in a control deck this is huge. I hate when they print win condition cards like this. If a shaman plays this, you either have a counter right away or you lose. 10 mana, summon 15/18 worth of stats that require a hard board clear. Rogue and druid have no counter for that. And remember, shaman has tons of spell discover to find this late game.
Thing is, by the time you play this card, your opponent either already has a good enough board to deal with it, or you've already won the game. It's only a win-more card, not really much else. Also it's 13 mana. Having 7 mana in the later turns feels really bad.