When you return to Vigil's Keep throne room after completing the three major quests - "Shadows of the Blackmarsh," "The Righteous Path," and 'Last of the Legion" - Senschal Varel will have one final quest for you. Speak to him when you are fully geared and have spent most of your money upgrading runes, bulking up your store of potions, and swapping inventory equipment until your four main characters have the best of the best.
News reaches you that a darkspawn army advances on the city of Amaranthine. It will take too long to mobiize the keep's forces to stop an attack, so you volunteer your party to intercept. Choose your party wisely. depending on your forthcoming actions, these may be the final party members for the rest of the game.
Go straight to Amaranthine, where the battle has already begun. Darkspawn swarm the city, and you enter combat as soon as you arrive at the front gate. Fight through the genlocks, hurlocks, and childers to save the citizens at the gate. If your party starts to get flanked, retreat to your starting location where you can only be attacked head on.
Once the first combat ends, Constable Aidan approaches and explains that the city may be lost. Darkspawn somehow poured into the city during the night and ravaged the population. The city guard is in disarray and under constant attack.
Before you can make a decision, a messenger from the Architect arrives and informs you that the Mother's army moves on Vigil's Keep. While you're occupied here, the Mother wants Vigil's Keep destroyed.
You now have a choice: save Amaranthine or save Vigil's Keep. You cannot save both. Despite the current state of affairs in Amaranthine, it can be saved with some hard work, or you can give the command to burn the city to the ground and race back to Vigil's Keep to support your fellow grey Wardens. If you choose to save Vigil's Keep, read the next section. If you choose to save Amaranthine, skip the next section and read the following section.