Nihilum is really on fire these days. Fresh from seven world firsts in the past week or so, they have now achieved the world-first kill (on the live realms, anyway) of the Essence of Souls, a Black Temple boss. No details have emerged about the fight that I've been able to dredge up, although WoWWiki speculates that the boss has a different phase for each of its three different faces.
In their comments thread, Nihilum deem the Essence of Souls "the hardest encounter we've met so far in WoW, ever" and "likely the hardest and most intense encounter in the history of world of warcraft," so my guess is it was probably pretty hard. Congrats! And in an unusual turn of events, they actually got their loot upon killing the boss:
MMO-Champion has this boss's full loot table, thanks to PTR kills.