Battle of the Arbor Wilds

The Battle of the Arbor Wilds was one of the last major engagements during the War against the Elder One.

Background
Having being defeated at the siege of Adamant Fortress and with his plans to get control over Orlais thwarted by the Inquisition at the Winter Palace, the darkspawn magister Corypheus uprooted his strongholds and concentrated his remaining forces on scouring the Arbor Wilds for elven artifacts that could help him to reach the Fade. Morrigan, a veteran of the Fifth Blight, Empress Celene's arcane advisor and newly appointed liaison to the Inquisition, suspected that Corypheus was seeking an eluvian, a magically elven mirror that could allow him to enter the Fade without need of the Anchor, allowing him to achieve through magic what political deceit and military might had failed to deliver.

Mustering the allied forces of the Orlesian armies, as well as their own forces, the Inquisition marched on the Arbor Wilds post-haste. As way to gain time for the allied forces to reach the battlefield, Leliana's spies were sent beforehand to sabotage the operations of the forces of Corypheus.

The Battle of the Arbor Wilds
At the Arbor Wilds, the forces of Orlais and the Inquisition faced Corypheus forces, and soon the allied armies was overpowering their rivals. However, yhe indiscriminate devastation of the forest during the battle awoke ancient elven sentinels, who were protectors of the Temple of Mythal located in the region. Those elves were hostile to both factions and engaged them in battle as well.

The Inquisitor, along with Morrigan and a few companions, went to the Temple to stop Corypheus before he could get eluvian. As they traveled across the wilds fought through the blockades, fighting against the forces of the Elder One and squads of sentinels, managing to catch up with Corypheus as the darkspawn was about to enter the Temple. While battling a group of sentinels protecting the entrance of the Temple, Corypheus was then seemingly destroyed by a magical barrier, only to be reborn in the body of a nearby Grey Warden. The Inquisitor's party quickly took refuge inside the temple as Corypheus summoned his red lyrium dragon to attack.

The Inquisitor's party then navigated the temple, either fighting through the sentinels or allying with them, before confronting and besting Corypheus' general, that may have been Raleigh Samson or Calpernia.

Afterwards, either the Inquisitor or Morrigan drank from the Well of Sorrows, which was the key to the resident eluvian, to render it useless. At the same time, Corypheus managed to storm his way into the temple and killed nearly all the sentinel defenders. The Inquisitor's party used the eluvian to return to Skyhold as a mysterious feminine entity arose from the Well, which blocked Corypheus and smashed into the eluvian, destroying it.

As Corypheus and his dragon fled the field, the Inquisition and Orlesian forces were able to finally rout the magister's army, nearly wiping it out.

Aftermath
The defeat at the Battle of the Arbor Wilds was so devastating for Corypheus, that he had no other choice but to challenge the Inquisitor on its own, just with the help of his dragon, this time at the Temple of Sacred Ashes. However, thanks to the knowledge gained from the Well, the Inquisitor was able counter Corypheus' dragon with their own (either Morrigan transforming herself into one or summoning the mighty Guardian of Mythal). After a long and grueling fight, Corypheus was finally slain putting an end to his threat for good.