Woo, yeah, new raid tier in 6.2, high five!!
edit - Loads of Info Here
Bosses have been revealed (click here for a gif of most of them - thnx @Gourmandise):
Hellfire Assault (loads of fel iron orcs)
The Iron Reaver (hell yes)
Hellfire High Council (Jubei'Thos, Dia Darkwhisper, Gurtogg Bloodboil oh shi-)
Kormrok (fel magnaron)
Kilrogg Deadeye
The Monstrous Gorefiend - (a mutated form of Teron'gor aka Teron Gorefiend, Auchindoun was merely a setback)
Shadow-Lord Iskar (not good ol' Iskar!)
Fel Lord Zakuun (a big elite Felguard type)
Xhul'horac (some kind of fel Nhallish)
Socrethar the Eternal (Shattrath was merely a setback - comes back in one of those Draenei-robots, but fel powered)
Tyrant Velhari - lady draenei anti-paladin
Mannoroth (starts out as a reanimated skeletal Mannoroth, WoD opening cinematic merely a setback hurrr)
Archimonde (!)
Supreme Lord Kazzak - World Boss yo
User Zieprakis has a thread here for new models.
Pretty neat stuff. No word on it being the last raid though. And it appears Archimonde is the final boss - would be a little strange if he were the last boss of the expansion. So there might be another raid tier to look forwards to.
More info as it becomes available. Hopefully lots of new creature models as well.
edit - whoa now
Had a few too many souls there, Teron'gor?