Siege of Adamant Fortress

The Siege of Adamant Fortress was a major engagement during the War against the Elder One.

Background
When every Grey Warden in Orlais began to hear the Calling—a bluff elicited by the darkspawn magister Corypheus—, the Grey Wardens grew desperate and sought out allies to help them accomplish their mandate before they perished, including summoning all the southern Grey Wardens to Orlais. Lord Livius Erimond, a Venatori magister, convinced Warden-Commander Clarel de Chanson to use blood magic techniques to raise a demon army and invade the Deep Roads and kill the Old Gods before they woke. Unbeknownst to the Grey Wardens, the demon binding rituals Erimond taught the Grey Warden mages enslaved them to Corypheus, who would use them to conquer Thedas.

A senior Warden (that can be either Jean-Marc Stroud, Alistair, or Loghain Mac Tir, based on player's choices from past games), that was researching about the red lyrium at the time, at behest of Hawke, the Champion of Kirkwall, was branded a traitor for protesting Warden-Commander Clarel's plan to use the blood magic rituals and was forced to went into hiding.

Hawke was later contacted by their old friend Varric Tethras, now a member of the Inquisition, who introduced them to the Inquisitor. After answering the Inquisitor's questions about Corypheus and how he may be related to missing Grey Wardens, Hawke and the Inquisitor rendezvoused with the Warden contact in a smuggler's cave in Crestwood to see what his investigation uncovered. The Warden contact explains that he was investigating if Corypheus could have survived fatal wounds just like an Archdemon can. Yet in the middle of his investigation, every Grey Warden in Orlais began to hear Calling. Believing that their end is near, Warden-Commander Clarel planned on using Erimond's blood magic ritual, believing that this course of action would end all Blights before all Wardens perished.

The Warden ally, Hawke, and the Inquisitor later scouted the ancient Tevinter ritual tower in the Western Approach to investigate a Grey Warden congregation and learned the truth of the Venatori's blood sacrifice rituals. As Erimond escapes to Adamant Fortress, the Inquisitor determines that the best course of action is to lay siege to the fortress.

Due to the fortress' age, it would not withstand modern siege equipment, which was provided to the Inquisition by Lady Seryl of Jader, and Leliana provided tactical information to limit the field of battle. The Inquisition then laid siege to the fortress, only to preoccupy the Wardens while the Inquisitor's goal was to brought down Erimond. Hawke and the Warden ally joined the Inquisition during this mission.

The Siege
Led by Commander Cullen Rutherford, the forces of the Inquisition storm Adamant Fortress, allowing the Inquisitor and their companions to infiltrating the walls. However, the Grey Wardens assembled there fought back, and a battle between the Wardens and the summoned demons against the Inquisition's soldiers ensued.

The Inquisitor and their allies arrived at the fortress' heart as the Wardens were in the midst of summoning a Nightmare demon. The Inquisitor confronted Erimond and Clarel about the truth of the demon binding ritual and attempted to sway the Wardens against Erimond. Erimond then summoned Corypheus' red lyrium dragon to deal with the Inquisition's forces. Seeing the appearance of the dragon, Clarel realized she was manipulated and turned on Erimond in the spot.

As the dragon attacked the fortress a massive battle between the Inquisition's forces and some Grey Wardens that had joined them after learning the truth, and the brainwashed Grey Wardens and their summoned demons ensued. Meanwhile, Clarel inflicted her wrath on Erimond but was killed while fighting Corypheus' dragon. Clarel's last act was casting a spell that subdued the dragon but its crash caused the ground beneath them to crumble, resulting in the Inquisitor's party, Hawke and the Warden ally to fall off the ramparts. The Inquisitor used their mark to open a rift to physically transport the party to the Fade.

The battle in the Fade
In the Fade, the Inquisitor found some kind of spirit that posed as the late Divine Justinia V, who briefed them about the nature of the Fade realm they are in, the demesne of the Nightmare demon the Wardens were trying to summon to the waking world, and that the Inquisitor must recover the memories it took from them while battling the demons who served the Nightmare. As the Inquisitor recovered these memories, they remembered how Divine Justinia was killed by Corypheus.

Once Hawke realized that a few Grey Wardens had a hand in sacrificing the Divine, they became incensed and accused the Grey Wardens of being out of control and need to be checked, alienating the Warden ally, who argued that the Wardens were most likely mind controlled by Corypheus and that Hawke themself had caused much chaos by causing the mage rebellion.

The spirit the led the Inquisitor's party to a rift that will allow them to return to the waking world, but the Nightmare was preventing their escape. The spirit sacrificed itself to weaken the Nightmare and the Inquisitor's party defeated the Aspect of the Nightmare after a difficult battle. However, the Nightmare demon, while weakened after having losing its Aspect, wasn't defeated and one ally had to sacrifice themself to distract it and allow the others to escape. This ally was either Hawke or the Warden ally.

Aftermath
With the Nightmare demon weakened, the fake Calling and Corypheus' hold over most Wardens was broken, and the demons were quickly dealt with by the combined forces of the Inquisition and the Grey Wardens. After the battle was over, the Inquisitor decided to either exile the Grey Wardens from Orlais to protect them from Corypheus' influence and as a way for them to atone for their misjudgment, or to ally with them and allow them to rebuilt their forces, bolstering the Inquisition's power. The battle also hardened the Inquisition's soldiers, making them battle-ready veterans. Afterwards, the Inquisitor may have ordered the Bull's Chargers to destroy Adamant Fortress.

Meanwhile, Corypheus had suffered a crippling defeat. He lost his demon army and had only a few dozen enthralled Wardens under his control. Erimond, who survived the battle, was taken captive in the fighting's aftermath and handed over to face justice at the Inquisitor's pleasure.