Host of the Eternity King
The Host of the Eternity King is one of the armies released as part of The End Times supplements, appearing in the End Times: Khaine splatbooks. In short, the Host of the Eternity King is stemming from the reveal that Malekith was always supposed to be Asuryan's chosen Phoenix King, he simply wimped out one second too early. Because of this, and the severe fuckery that Nagash and Chaos are up to, all three forces of elves -- Asur, Druchii, and Asrai -- are having to unite as one if they want to have any hope of surviving the End Times.
In canon chronology, the Host of the Eternity King is the end result; initially, the elves fracture into the Aestyrion (High and Dark Elves under Tyrion, avatar of Khaine) and the Host of the Phoenix King (loyalist Dark Elves, High Elves and Wood Elves) until Malekith defeats Tyrion.
The Host of the Eternity King contains all units from the three armies, with the only lacking areas being special characters, since a lot of them got snuffed during the build-up leading to the formation of this army.
Incidentally, this makes them the third army based on "mix together formerly separate armies", after Undead Legion and Legions of Chaos..