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.
After the destruction of the Ruby Rod, Duumal would remain a hotspot for fighting in the Hells, with even more warlords beginning to claim territory. After the Years of Fiendish Blood concluded, these warlords entered into a loose peace, uniting to combat the newly returned demons.
Gehena War Profiteering
With the conclusion of the Blood War, the Yugoloths of Gehena lost their largest clients. But the mercenary fiends of Gehena were able to quickly pivot, offering their services to the newly risen factions in Duumal and charging premiums for their aid. Some Yugoloths though would seek to strike out their own as independent warlords themselves, with many of these warlords eventually joining the Sinners league.
Dis
Minarous
Phlegethos
The many slaves and prisoners of Phlegethos would rise in revolt, encouraged by the words of the Storm Giant sorcerous Thrya Cloudbreaker. These former slaves would begin to overthrown their former archdevil masters, holding mass executions. many archdevils were forced to flee Phlegethos or else risk death at the hands of their now liberated slaves.
Stygia
Stygia would become ravaged by a massive storm conjured by the Frost Giant Druid Hrothgar Icevein that would begin to tear the glaciers and ice flows of the layer apart. Most major structures within Stygia would be destroyed, with the only survivors existing on the far outskirts of Stygia outside of the storms reach.
Malbolge
Malbolge would be invaded by the draconic armies of Barroth the Unredeemable, a fiendish dragon who sought to gain a foothold in the Hells for Tiamat. After hearing about the draconic devil Delphyne and her partner, the ruler of Malbolge, Philani, Barroth and his forces would travel to Malbolge to claim the title of "Dragon of the Hells" for himself to to take control of Delphyne's own draconic army. Barroth would gain control of a portal to Malbolge in Duumal called Glasya's Folly and use it to enter into Malbolge, also claiming for himself the nearby fortress of Drossmere as his base of operations. Both armies would engage in a brutal war with both Barroth and Philani sending diplomats across the planes in search of allies to aid them in their war.
Barroth would gain an edge in the war by forging an alliance with Dispater an adding iron Devils and war constructs to his army. Though his advances would be short lived due to the arrival of the armies of the archangel Gabriel whose armies had been traveling through the Hells layer by layer, slaying any devils they could find.
After Barroth and Gabriel's deaths, Philani would regain control of Malbolge and Delphyne would subjugate Barroth's draconic armies.
Maladomini
During the Infernal insurrection, Maladomini would fall into chaos as a scramble for the crown of Maladomini ensued, egged on by the Cult of the Shattered Mountain. It quickly turned into king of the hill of filth with devils fighting all across the filth filled city streets to climb the mountain of dung in the layers center to dethrone the current ruler Baalzebul.
Cania
The archdevil Mephistophales would engage the Storm Giant sorcerous Thrya Cloudbreaker in arcane battle above the skies of Cania. But the Storm Rune granted to her by her goddess gave Thrya immunity to Mephistophales' magic, and after a full week of battling, Thrya was able to force Mephistophales to retreat, allowing her forces to pillage Cania for all its arcane relics and secrets. In the chaos mages from across the planes would travel to Cania to join in the stealing of arcane artifacts, leading to hundreds of long lost dangerous and cursed magical relics being released upon the realm.
Nessus
Thanks to a deal struck between the archduchess Glasya and the Cult of the Shattered Mountain, Glasya was able to avoid the chaos that consumed the rest of the Hells. She would host members of the Cult within her fortress, Malsheem, and give them access to the tenth layer of the Hells, Hellmar. She would later claim the Hill Rune after the death of the Hill Giant Angrboda and join the remaining Ordained in their attempt to resurrect their goddess Othea. But she would be slain during the ritual by the heroes known as the Hellraisers.
Hellmar
The Three Invasions
The Invasion of the Material Plane
In a bid to harvest new souls to maintain his power, the Lord of the Second layer of the Hells, Dispater, sent his armies of Iron Devils into the inner planes. These constructs were able to bypass the Pact primeval and harm mortals, and so began a campaign of destruction across the Material Plane, Feywild, Shadowfell, and the Elemental Planes. Mortals killed by iron Devils either had their souls sent to Dispater himself or had their souls transformed into new Iron Devils.
But soon after, Dispater's iron armies would be met on the battlefield by the angelic armies of the archangel Gabriel who used Dispater's war of aggression as an excuse to finally confront the devils. But Gabriel cared more about killing devils then saving mortals, and his armies ended up inadvertently killing as many mortals as did Dispater's.
Even after Dispater's death, other devils would use similar tactics to harvest souls and would lead their own campaigns into the Material Realm to massacre mortals.