The Infernal Insurrection (sometimes referred to as the Third Infernal Emergence) refers a period of chaos, between 925 PR and 943 PR, throughout the Ten Hells and the lower outer planes after the River Styx dried up. Most notably it involved a war between the Ten Hells and Mt. Celestia, destabilization throughout ever layer of the Ten Hells, and the disappearance of Demons and the Abyss. While the River Styx was replaced by the Passage of Souls in 926 PR, there were still many small pockets of instability left throughout the lower planes.
The Loss of the River Styx
In 925 PR, in a plot orchestrated by the Lord of Greed Mammon, a group known as the Cult of the Shattered Mountain with the aid of the Demon Lord Ratatrosk slew the ferryman Charon and used Charon's oar to destroy the River Styx. Charon was under divine protective magic that prevented any creation of the Gods from harming him, but since Demons weren't made from the gods, Ratatrosk was able to kill him.
With the River Styx destroyed, souls were no longer able to pass into their afterlives, and were trapped in the Ethereal Plane. This led to a period of panic in the Hells as no more new souls meant no more new devils.
The Disappearance of the Abyss
Another consequence of the Styx's destruction was the disappearance of all Demons from Quelmar, excluding the Demon Ratatrosk. All demons simultaneously vanished and all portals that led to the Abyss closed. Any attempts to contact the Abyss or travel there by planar means were also unsuccessful. Without the River Styx has a connecting thread, the Abyss had been cut off from Quelmar and the outer planes.