It's a little blurred but its a 2 mana 2/2 with deathrattle into 6 mana 6/6 with deathrattle into a 10 mana 10/10.
So basically a creature card that can be summoned three times. It's cool in practice. If meta changes away from Pirate aggro and decks that attempt to play their hands quickly then would consider this.
Bad card, will never see play. 2/2 for 2? Meh. 6/6 for six is meh and 10/10 for 10 is unplayable. It would be cool if second and third phase would have some cool effect, like flame something, but this is just lost opportunity.
Arena material. That's it. I doubt it's good enough to be played with N'Zoth unless mage get something crazy to healt to replace Reno.
In a Control Meta (Maybe one day TM), it could be playable. I mean, a 2/2 ->6/6 which are both summoned from N'Zoth result technically in 3 vanilla 10/10. M'well, it's something. But even with that I doubt it's still something for the same reason Anub arak was never played seriously.
Tempowise it's weak, but I wouldn't call it bad. It's a control card, that allows you to build your deck a little bit differently, allows you to put a little bit more removal spells and acts as a curving out win condition in some sorts. Quite interesting card.