The Infernal Insurrection

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.

Chaos in the Hells

Duumal

With demons gone and the Blood War over, the armies of Hell were left without a purpose and quickly broke up into many waring factions hoping to grab whatever power they could in the confusion. The three most prominent factions to emerge was Bel's Militia, commanded and organized by the Pit Fiend Bel, one of Duumals former rulers, the Sinners League, a loose alliance of warlords and mercenaries from the Blood War, and the Fingers of Barroth, draconic devils serving the evil dragon Barroth.

Bel's militia quickly gained an edge in the fight by purchasing weapons from Dispater, the Lord of the second layer of the Hells, Dis, and with their control over the Bronze Citadel, Duumal's largest population center. His larges assest was a massive iron bull war machine called the Soulnaught. Infighting within the Sinner's League and a betrayal by one of their new members, the Hill Giant Angrboda, allowed Bel to ambush them at Bloodmaw Keep and scatter the forces of the Sinners League. With the Sinners League out of the picture Bel was quickly able to expand his influence over Duumal, bring more of Hell's armies under his command, and even campture the current archduke of Duumal, Elytria.

During Elytria's execution, Bel and his forces were ambushed by the remaining members of the Sinners League and an adventuring party known as the Hellraisers. This battle quickly turned sideways with the invasion of the angelic armies of the archangel Gabriel as well as the arrival of an Inevitable from Mechanus and infernal archmages from Cania. During the battle Elytria was saved and Bel was slain, but Angrboda stole Elytria's Ruby heart and fled before the armies of Gabriel could arrive in full force.