Scouring

The Scouring is an event mentioned by Xal'atath, along with the Cataclysm and the Great Sundering, that caused great upheaval in the land.

Speculation
The Scouring likely refers to the fall of the Black Empire by the titan-forged armies of the Pantheon, who toppled the Old Gods' temples, destroyed their cities, eliminated almost all aqir and erased their presence from the surface of the planet.