The Desolate Host
The Desolate Host is the sixth boss of the Tomb of Sargeras raid instance and a three-in-one. Ever dreamed of fighting an Engine of Souls, Soul Queen, and Tormented Souls all at once? Well, then your dreams almost come true: you fight the first two simultaneously, while the third appear to substitute them.
The Legion commanders found it a good idea to construct the Engine of Souls on the site of Elvian sacred burial ground, which should not come as a surprise to anyone who knows how wicked The Legion is. The machine draws upon the energy of the dead and twists them into some kind of abominations. And as anything, resurrected in such a manner, once Night Elves, now monsters do not consider the living welcome guests.
Tactic-wise, you will have to deal with two bosses with shared HP during Phase 1: the raid will have to divide in half, and to fight in both the Corporeal and Spirit Realms with two different things (things indeed, as one of them is not even a creature). Phase 2 is about fighting Tormented Souls, but the action is still happening in two separate worlds. Probably, one of the most exciting encounters of the raid.