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Was announced at PAX East during Blizzard's panel (as everyone expected)

There's going to be 5 wings, with a total of 17 bosses (totalling to 31 new cards overall).
The first wing was confirmed to be Blackrock Depths, with The Grim Guzzler, The Arena and Dark Iron Emperor as the bosses. The second wing was stated to be Molten Core (presumably with Ragnaros as the last boss there).

I'd take a guess at LBRS, UBRS and BWL being the final 3 wings given they talked about Nefarian as well as Ragnaros and the conflict between the two groups.

Many of the new cards have been said to be related to dragons in some way (for example, Blackhand as below)






All I can say is rip zoo

dat 4 mana 5/6