Siege of Bastion's Keep

"You thought you were so clever, that you had outwitted us all. One by one, our brethren fell into your trap, but not me. I defy you! I now know the Black Soulstone is the key, and it shall be mine. Soon, my armies shall pour forth from the Shattered Mountain, ravaging this world and all hope of resistance. My minions will find the stone, wherever you choose to hide it. Then, at long last, Azmodan shall reign as the Prime Evil!"

- Azmodan's declaration

The Siege of Bastion's Keep was a battle that took place in the year 1285.

Opening Moves
Azmodan emerged in Sanctuary with the intent of obtaining the Black Soulstone. Leading the Army of Sin, his forces emerged from the Arreat Crater. The crater served as a staging ground, and his army marched south.

By this point, the keep's garrison had been reduced from an intended 3000 to 1000. In contrast, the Army of Sin's numbers were practically without limit, as more demons emerged from Hell to join it with every passing hour.

The demons' assault was relentless. They seemed without number, and kept up their attack through both night and day, while Azmodan's war machines bombarded the walls. Morale among the besieged defenders dwindled, The demons were able to take the Stonefort, which prevented the catapults from being raised from their berths below. During the siege, a Siegebreaker Assault Beast was unleashed upon the fortress, and while it fell, it took over 100 men to bring it down.

At some point, a request for reinforcements was made of Westmarch.

On the ninth day of the siege, Commander Calderos decided that a "righteous show of force" was what was needed to send Azmodan's army into retreat, and intended to lead a charge from the fortifications the following day. Come the tenth day, Calderos led his charge, and he and his men were slaughtered at the gates.

Defeat neared. Bastion's Keep was close to falling, and Azmodan's minions knew it, as many demons paused in their assault to feast upon the flesh and blood of their mortal foes. Azmodan called an end to their indulgence and ordered a final assault, wanting the keep to fall before the next dawn. On the tenth day of the siege, Tyrael and his allies arrived. By this stage, the keep's soldiers were scattered, the original bastion of 1000 reduced to 300, and Azmodan was throwing everything he had against the keep. Legions of demons crashed against the walls, sending tremors through the bones of the fortress, and fiery projectiles rained down against the walls. That the damage from twenty years ago had yet to be repaired didn't help matters. Tyrael, taking command of the keep's defenders, ordered The Nephalem to light the keep's signal fires, which served to boost the defenders' morale. Azmodan was aware of the Nephalem's presence, but kept up the attack nonetheless.

The Tide Turns
"You heard the angel’s command! Let the call sound forth&mdash;Bastion’s Keep shall not fall!"

- One of the keep's defenders

The Nephalem made their way across the keep's battlements successfully lighting the signal fires even as demons spilled over the walls. The Nephalem met with Sergeant Dalen, who informed them that the catapults could break Azmodan's lines, but couldn't be raised due to the presence of demons on the walls. Afterwards, the Nephalem, with the aid of the keep's soldiers, was able to raise the fortress's catapults into position. With that done, projectiles rained down upon Azmodan's army. His lines fell into disarray almost immediately.

Azmodan mocked The Nephalem, revealing that while the catapults were being raised, his forces had breached the keep in its lower levels. The Nephalem and the stronghold's defenders were able to repel them, and The Nephalem headed into the depths to seal the breach in the lower walls. There were few defenders down there, just soldiers gathering supplies. In this, they succeeded, defeating the demon Ghom.

A messenger arrived from Westmarch, informing Tyrael that King Justinian would not send reinforcements to fight "imaginary demon lords." Tyrael bitterly commented that the king might believe when his kingdom was burnt to ashes around him.

The Nephalem proceeded to deal with the new threat as demons entered the keep through its lower levels, even killing Ghom, one of Azmodan's Sin Lieutenants. However, while the demons had been repelled from the fortress, Azmodan's war machines continued to bombard it. The Nephalem headed out onto the battlefields, destroying the constructs. However, demons continued to emerge from the Arreat Crater, prompting The Nephalem to descend into the crater to confront Azmodan. By this point, Azmodan had dredged up Sin Hearts from his realm to empower his legions.

The Nephalem fought their way through the crater, killing the Sin Lieutenant Cydaea, and destroying the Sin Hearts. Finally, Azmodan himself was defeated, ending the siege days after they had arrived.

Aftermath
By the end of the siege, the bodies of Azmodan's legions littered the battlefield. With the news of Azmodan's defeat, many of the keep's defenders rejoiced, believing that they could head home. However, Victory was short lived, as Adria revealed her true colors, turning on the defenders and killing many of them. While Azmodan was defeated, Diablo, reborn as the Prime Evil, was let loose upon the world. Many of the survivors were demoralized not only by Adria's betrayal and the death of Leah, but by the capture of Auriel, the Archangel of Hope, which brought despair to the High Heavens and Sanctuary until her rescue by the Nephalem. While Diablo was defeated, Bastion's Keep was left in a precarious state, as all of its food stores had been spoiled by Ghom. Sister Frances wondered how the garrison would cope over the next season.

During the battle, Tyrael commented that even Imperius would have been hard pressed to have held the keep against Azmodan's forces, and that the world owed its defenders a great debt.