Redditor eke00372 uploaded a video of Argus the Unmaker from PTR. Blizzard opened Antorus for testing and a guild was able to access the raid's final boss, even though testing should have ended with Aggramar. The fight is still very unfinished. The boss can be CCed and isn't even an elite.
After defeating Aggramar, you talk to Magni and get teleported to the Seat of the Pantheon.
The fight takes place in an area called Seat of the Pantheon.
Multiple rewards await after you defeat Legion's final boss.
Scythe of the Unmaker (Polearm Weapon Transmog).
Shackled Ur'zul mount drops from Mythic Argus.
Kirin Tor Summoning CrystalKirin Tor Summoning Crystal is obtained when you complete Blood of a Titan.
Insignia of the Grand Army from completing The Death of a Titan.