Age of Demons

Roughly 100 million years ago until 1 million years ago
With the barrier between the material plane and other extraplanar locations torn, demons and various fiends pour into Terrus. Their rampage destroys nearly all semblence of civilization. The mortal races join together and rally against them. During this time, the aboleths also escape their prison. Using their powers, the aboleths turn the tide against the demons and realign the planes.