Alphinaud departs to inform Alisaie of what you have learned, and you go to meet up with them.
This quest is available if you have completed the quest Honorable Heroes.
Starting the Quest
Talk to Alphinaud in Southern Thanalan to begin the quest.
Speak with Alisaie
Go southeast and go through the gate behind the Brass Blade, then go northwest to the quest icon on the map. Talk to Alisaie and there will be a cutscene.
Speak with Alphinaud
There will be a glow around Alphinaud, indicating an instanced battle. Check on Alphinaud and there will be a cutscene. This one is important, so it is recommended to watch it either now or via the Unending Journey in an inn room. After the cutscene, the battle will ensue.
Solo Duty: One Life for One World
Defeat the Devout of Darkness, followed by the Knight of Darkness. Throughout the battle, you will need to dodge circle attacks. You will sometimes be marked with a red marker over your head. Go to an edge of the area and you will be hit by Skydrive, which leaves a persistent puddle. Move away from the puddle to avoid damage.
After a time, the fallen warriors will be revived. Repeat the strategy above, as many times as necessary, until you are chained. There will be a cutscene. This one is important, so it is recommended to watch it either now or via the Unending Journey in an inn room.
After the cutscene, the battle continues.
During this phase of battle, there will be more attacks to dodge, including a series of cone attacks from the Warrior of Darkness. Stand on one of the cones that appears later than the others, then when an adjacent cone disappears, move to where the cone disappeared from.
Whenever the Blade of Light becomes interactable, interact with it. If any enemy is interrupting you, first defeat the Devout of Darkness if she is alive, then defeat the enemy that is interrupting you.
When the Ranger of Darkness tethers to Alisaie, first defeat the Devout of Darkness if she is alive, then defeat the Ranger of Darkness. Then interact with the Blade of Light as soon as possible. As before, if an enemy is interrupting you when you try to interact with the Blade of Light, defeat the Devout of Light, followed by the enemy interrupting you, so that you can interact with it. The Warriors of Darkness might be revived before you have a chance to do so, so repeat the strategy until you are able to interact with the Blade of Light.
After the battle, there will be a cutscene. This one is important, so it is recommended to watch it either now or via the Unending Journey in an inn room.
Speak with Alphinaud Once More
Talk to Alphinaud to complete the quest.
Next Main Scenario Quest
After you complete the quest, talk to Alphinaud to accept the next quest: An Ending to Mark a New Beginning.