Watch the cutscene and get ready.
FINAL BOSS - Archdemon
This fight is broken down into phases; every 25% the Archdemon will move to another phase. Summon your armies but leave a melee and ranged unit for later. It uses some area of effect attacks: a stunning roar, flame breath, bites, and tail swipes. It takes flight to move around and does so often so just adjust and play it like the other dragons you have fought.
When the second phase starts it will use Vortex , an attack that deals minor damage but afflicts anyone it hits with attack and defense debuffs. Just continue like it was the first stage being wary of the Vortex.
The third phase starts at half health and the Archdemon takes flight. Use the ballistas and ranged attacks to deal damage to it. Summon the ranged unit you held earlier and take notice of the Darkspawn that pour into the area. Have your mage use some area of effect spells to quickly wipe them out and then switch back to the Archdemon.
Phase four starts at 25% and the Archdemon summons more Darkspawn. Use the last unit that you held now to focus on the Darkspawn while you deal the damage to the Archdemon. Whatever choice you made (Morrigan’s offer or not) at the end of the game will decide the ending. Enjoy the ending!
A Dark Promise
Defeated the Archdemon and, through a dark ritual with Morrigan, spared your own life.
The Ultimate Sacrifice
The ultimate sacrifice was made in defense of Ferelden.