It is time to prepare for the final confrontation with Corypheus.
The quest is automatically triggered after finishing the quest The Final Piece.
Head to the War Room in Skyhold to begin this quest. It costs 0 Power and is recommended for levels 16-19.
When you first arrive at the Valley of Sacred Ashes, you will immediately encounter Corypheus and after some banter back and forth, his red lyrium dragon appears. Morrigan, in dragon form, will engage his dragon in battle if she was the one to drink from the Well of Sorrows. Alternatively, if it was the Inquisitor who drank from the well, the Guardian of Mythal will appear to fight it instead. The cutscene ends and you engage with Corypheus.
In the first stage of the fight, he will summon shades and teleport away from the fight.
Follow Corypheus upstairs to the next "floor" where the next encounter will happen. At about 75% health he'll move away again.
Follow Corypheus further into the ruins. There is a Supply Cache on the way, where you can resupply, but you cannot save the game. At the next encounter Corypheus will summon more shades to help him. At about 50% health there will be a cutscene, where you can see his dragon fighting your ally.
The next stage is a fight against Corypheus' Red Lyrium Dragon, which starts with only 50% health. This battle will end when the dragon's life drops to about 10%. There will be another cutscene, in which your Inquisitor will finish the dragon.
Follow up the stairs, where you find another Supply Cache on the way. Then you will encounter Corypheus again who is still down to 50% health. During the fight Corypheus will heal himself back to the 50% life, but his attacks will weaken after this. At about 5% health the fight will end and the final cutscene is triggered.
You will return to Skyhold where you celebrate with your companions and advisors. All doors will be blocked by people, so you can only leave the party to your private rooms. If you do that another cutscene will be triggered with your romance choice, if you have one.
After the epilogue plays you will be back at Skyhold and are able to finish remaining side quests.
- Most of the quest names in the main plot refer to specific verses of the Chant of Light. "Doom upon all the world" refers to the Canticle of Threnodies 8:13, which in turn references the Tevinter Imperium's invasion and subsequent corruption of the Golden City - the "Second Sin" Corypheus was partly responsible for.
- There is a bug upon completion that shows a ghost hall with next to no sound save background and your character the Inquisitor, most likely due to a different hall design other than the basic decor. Otherwise the cause is unknown.