Lore Spotlight: Argus the Unmaker
Argus, "the emerald star", was originally the homeworld of the eredar. Like Azeroth, this world was uniquely special, as it contained the world-soul of a sleeping Titan - a race of godlike, planet-sized cosmic beings of Order. When the planet fell to the Burning Legion, the world-soul inside was twisted and corrupted, used to fuel the Legion's infinite army
To defeat the Burning Legion, we had to sever this connection to the world-soul. In the Antorus, der Brennende Thron
raid, the Pantheon of Titans transported Argus to the Seat of the Pantheon, where we faced and killed the tormented world-soul. Though this put Argus out of his misery and saved Azeroth from the Burning Legion, it seems the Dreadlords of the Legion were playing the Jailer's, or perhaps Denathrius', long-game.
Imbued with Death magic by the Dreadlords, we now know Argus's own death broke the Arbiter, throwing the Shadowlands into the chaos that we found it in when we arrived.