You have to return to the Athlum Castle for another cutscene.
The quest Bravery and Loyalty will permanently disappear after this cutscene once Emma leaves your party and before entering the Nest of Eagles! Finish it before she leaves the party!
Spoiler warning: Cutscene details.
Pagus informs David that the Nest of Eagles is where the battle is at. David divides his generals, some to go support Celapaleis, some to guard Athlum. However, Emma says she alone should stay and guard Athlum. David goes along with this and orders everyone else to march out.
Emma will leave the party to remain and protect Athlum. Rearrange your unions as needed.
The Nest of Eagles
Roeas leading the Conqueror's army.
The next target is the Nest of Eagles to stop the Conqueror's army from reaching Celapaleis.
A boss battle starts as soon as you enter the area. Check the location page for more details on the fight.
Once you are victorious, a long cutscene will begin.
Spoiler warning: Cutscene details: MAJOR SPOILERS.
With the battle at the Nest of Eagles won, the Conqueror attacks Athlum directly, by himself. He breaks open the gate to the town square, and Emma engages him in battle, telling the other soldiers to flee. After some swordplay, the Conqueror mortally wounds Emma using the Tablet. Then he binds the Valeria Heart and walks off.
Some time after this, Emma's body is on a table and the remaining Generals are mourning her. David has taken this very hard and has her sword with him.
Rush goes looking for him and finds him in the palace garden. David speaks of his childhood, how Emma was a mother to him. Then Emmy enters the scene, swearing allegiance to David now that her rite of passage is over.
Marina Sykes explains that Hermeien is the one plotting to use both Wagram and the Conqueror in order to start a war and become the new God Emperor. Specifically he wants to use Irina as the new Marion Marshall to control all remnants. Everyone sets out to rescue Irina.
PC: You can now have 5 Unions, 15 Units per battle, and 40 total Units X360: You can now have 5 Unions, 15 Units per battle, 6 Leaders per battle, 15 total Leaders, and 18 total Soldiers.