I am very disappointed. I was hoping for a powerful AoE that could give your opponent and you overload or give some freeze synergy for Freeze Shaman but we get a potential crappy evolve. With that being said, Hagatha and the Witch's Cauldron can spawn this too.
I voted as "Bad" for my disappointment and the lack of synergy with the other cards (other than swarming your foes).
There are so many badly stated legendaries in the game.. I see no reason to play this card over Thrall, Deathseer at least you are guaranteed to upgrade your minions, most of the time and for 2 mana cheaper.
Also palyign thrall on a favorable situation is tricky..
generally worse than evolve. Your paying 7 times the cost to get minions that could have worse stats than the minions you already have, and legendary minions generally have most of their value in battlecries.