Well, the preview for a second Old God card in Hearthstone has been released, and it's the most mysterious of the four known Old Gods, N'Zoth.
There's a link to the post that unveils it, where you'll get to see the gold, animated version of N'zoth, but here's the rundown:
10 Mana, 5 attack, 7 health. Battlecry: Summon all your Deathrattle minions that died this game.
Like all the Old God cards, N'zoth costs 10 mana, and like C'thun, he has a battlecry that seems more effective late in the game. The 5/7 body is not terribly impressive for the price, but I suspect that the mechanics of the various Old Gods are going to be based more on their powerful battlecries than their function as actual minions (though C'thun will probably be good for both if he's significantly buffed.)
Of course, even though lore-wise these characters should be more powerful than anything else we've seen in Hearthstone, they do have to be balanced as cards. You'll obviously need to play a lot of Deathrattle cards for N'zoth to be effective. I'm actually not sure how this fits flavorfully with N'zoth (wouldn't Yogg-Saron, the Old God of Death, be more of a Deathrattle-focused guy?) But you also need those minions to have died already to have their effect. Granted, minions don't often last too long in Hearthstone, but unlike C'thun, N'zoth isn't going to have a super-immediate effect.
That said, mass resurrection itself can be pretty powerful, and filling the board with minions is not going to hurt you. I do wonder what happens if you have more than six qualified minions that have died that game, how the game decides which ones get rezzed.