Reckoning of Hell

In the cosmology of the fantasy role-playing game Dungeons & Dragons, the Reckoning of Hell (often referred to as the Reckoning) was a civil war that shaped the political landscape of the Nine Hells into its current form. The Reckoning received its fullest treatment in the D&D sourcebook A Guide to Hell.

Prelude to the Reckoning
In the times before the Reckoning, the Lords of the Nine Hells were divided into two factions. Mephistopheles, lord of Cania, the eighth layer of Hell, led one faction. His allies were:
 * Dispater: lord of the Dis, the second layer.
 * Mammon: lord of Minauros, the third layer.
 * Geryon: lord of the Stygia, the fifth layer.

The opposing faction was led by Baalzebul, lord of Maladomini, the seventh layer. His allies included:
 * Zariel: lord of Avernus, the first layer.
 * Belial: lord of the Phlegethos, the fourth layer.
 * Moloch: Baalzebul's viceroy and ruler of the Malbolge, the sixth layer.

Baalzebul and Mephistopheles believed that by eliminating the opposing faction they could gain enough power to challenge Asmodeus, lord of Nessus, the ninth layer, and ruler of the other lords.

The war
The Reckoning was sparked by Baalzebul's faction. It began on Avernus: Zariel had prepared a great army, supposedly to lead an assault on the Tanar'ri in the Blood War. At the last minute, she diverted her army to Dis and laid siege on the Iron Citadel of Dispater. On cue, Moloch and Belial launched surprise attacks on Stygia.

Mephistopheles was caught off guard, and the initial war went badly for his faction. He sacrificed hordes of devils to buy time to organize a counter-offensive, then led his gelugon legions into Maladomini to attack Baalzebul, while Mammon's forces reinforced Dis. Zariel retreated to her home layer of Avernus, and Baalzebul called to his allies for reinforcements.

The war had shifted in Mephistopheles' favor, and the focus of conflict turned to Maladomini. All other battles were abandoned, and the two factions poured into the seventh layer for a final confrontation. Despite the horrendous casualties, both armies numbered in their millions. The lords were already thinking about deposing Asmodeus. At that moment, Asmodeus sprang his trap.

Unbeknownst to the other lords, Geryon had remained loyal to Asmodeus. With Asmodeus' help, Geryon had infiltrated the ranks of both armies. As the final clash grew imminent, Geryon blew his horn. On his sign, the pit fiend generals of the armies turned on their masters, revealing their ultimate loyalty to Asmodeus. The warring lords were overthrown by their own armies, leaving Asmodeus the undisputed leader of the Hells.

Aftermath
Asmodeus rewarded the pit fiend generals by giving them command over the baatezu armies of the Blood War. The generals are collectively known as the Dark Eight. The lords of the first eight layers retain personal armies and the Dark Eight rank below them. Ultimately, however, the Dark Eight remain completely loyal to Asmodeus, which ensures the vast majority of the baatezu are under Asmodeus' command.

Asmodeus surprised all by reinstating six of the lords almost immediately after their defeat. Asmodeus believed that his position would be more secure if he kept his chastised enemies busy ruling his fiefs under his watchful gaze, than if he banished them to plot against him in some dark corner of the lower planes. Only Moloch and Geryon were not reinstated as rulers.

In the time leading up to the Reckoning, Moloch's consort, the Hag Countess (known in 2E D&D as Malagard), encouraged him to join Baalzebul's alliance, suggesting he could dethrone Baalzebul after Mephistopheles and Asmodeus were removed from power. Unbeknownst to Moloch, the Hag Countess was in secret communication with Geryon, and she arranged for Moloch's armies to betray him. After Moloch's defeat, she counseled him that Asmodeus would respect his strength if he remained defiant. While the other lords fled before Asmodeus, Moloch alone stood his ground. For his impudence, Asmodeus replaced him with the Hag Countess as ruler of Malbolge. The Hag Countess banished Moloch and he disappeared from the Hells. Geryon, the only arch-devil to show loyalty to Asmodeus, was deposed and faded into anonymity. His layer was given to Prince Levistus, a former ruler of Stygia who'd been imprisoned in ice for eons as punishment for murdering Asmodeus' consort Bensozia. In spite of his return to power, Levistus remains encased in ice.

Mammon took on a new form after the Reckoning. His prior appearance as a bloated pit fiend was replaced by that of a giant serpent with a humanoid torso and head. According to A Guide to Hell, Mammon took this form to symbolize to Asmodeus that he'd become a "new devil"; according to the Book of Vile Darkness, Asmodeus bestowed this form on him, either to punish him or as a sign that Mammon had changed his traitorous ways. Mammon's new form resembles that of Geryon, and is possibly intended as a final insult to the forgotten former lord of Stygia. Asmodeus ended the relationship between Mammon and his consort Glasya, the daughter of Asmodeus and Bensozia. Asmodeus put Glasya in charge of the Erinyes, and since the Reckoning she has collaborated with the Dark Eight in that capacity.

Baalzebul, a former favorite of Asmodeus, was punished for his desire to rule the nine layers. Baalzebul's original form was a handsome humanoid with compound insect eyes; in the aftermath of the Reckoning Asmodeus transformed him into a giant slug with tiny malformed arms who leaves a trail of stinking waste.

After the Reckoning, Belial decided to retreat into the shadows. With Asmodeus' approval, he instated his daughter Fierna as ruler of Phlegethos. Belial remains the layer's true source of power behind the scenes.

Zariel was later overthrown and imprisoned by the pit fiend Bel, a general for the Dark Eight who'd served with distinction as Zariel's right hand.

Baator's current political landscape
Bel, Mammon, Levistus and the Hag Countess have no real allegiances:
 * The other arch-devils refer to Bel as the Pretender. They believe Amodeus allowed his coup because it gave him one less scheming lord to worry about. Bel is kept busy with his duties prosecuting the Blood War; he has attempted to ally with Baalzebul and Mephistopheles but has been rebuffed &mdash; the memory of his treachery against Zariel is too fresh in their minds.
 * Mammon, the first to betray his allies and abase himself before Asmodeus, is not trusted by the other lords.
 * Prince Levistus is obsessed with gaining enough power to escape his imprisonment. He loathes and is loathed by all the other lords.
 * Unlike the other lords, the Hag Countess is not a true baatezu and she achieved her power through guile rather than strength. She maintains her rank through military might and for the most part stays out of baatezu politics. Her main concern is solidifying her power base.

Dispater remains loyal to Mephistopheles and Belial remains loyal to Baalzebul. Mephistopheles and Baalzebul are still bitter rivals.

The fate of Moloch and Geryon
(Info from D&D modules The Apocalypse Stone and A Paladin in Hell.)

For centuries after his banishment, Moloch plotted his revenge, marshalling resources and gaining allies. When his preparations were complete, he went into hiding in the Prime Material Plane. He planned to planeshift to Baator and reclaim Malbolge at a prearranged time.

Unbeknownst to Moloch, a party of heroes stole an artifact called the Stone of Corbinet; its theft made shifting between the planes impossible. Moloch was stranded in the Prime Material Plane at the appointed hour. His forces on Baator were cut off from his orders and slaughtered by the Hag Countess.

Moloch pursued the party of heroes to exact vengeance. His ultimate fate was left at the DM's discretion.

According to A Paladin in Hell, it's likely that Asmodeus deposed and humiliated Geryon because he knew Geryon's desire to regain his former position would make him an easily manipulated servant.

On one occasion, Asmodeus sought to regain an artifact stored in a well-guarded temple by getting a servant to transport the temple to hell. With the promise of Asmodeus' aid in regaining his position as ruler of the fifth, Geryon agreed to invade the temple and retrieve the artifact when it arrived in Stygia. Geryon built an ice fortress called Coldsteel above the area the temple would appear, blocking all access points save through Coldsteel.

Geryon's plans were foiled by mortal adventurers who fought their way though Coldsteel and returned the temple to the Prime Material Plane. Geryon remains in Citadel Coldsteel, secretly raising an army to dethrone Prince Levistus.