! ATTENTION SPOILERS !
During your adventure in Fire Emblem: Three Houses, Edelgard opposes the Church. The default choice is to rally around dame Rhea and oppose the Empress of the Adrestian Empire. However, a last option is possible and allows you to unlock a completely new scenario in the game.
If you have met certain conditions, another choice may be available to you and you have the option of following Edelgard in his quest. In order to unlock the 4th hidden scenario, you must have chosen the Jet Eagles among the 3 houses available at the start of the game.
Here are the requirements to unlock the Edelgard storyline:
1 - During chapter 11 (month 2), talk to Edelgard:
Between 2/8 and 2/22, talk to Edelgard in the great hall during your free time in the Monastery. She offers to accompany her to the imperial capital to his coronation. You must accept his proposal to later unlock the hidden scenario. You won't get a second chance if you refuse. Be careful, if you respond to his request, this will put an end to your activities in the Monastery. We therefore advise you to spend all your points well and finish your activities.
2 - Achieve C+ support level with Edelgard:
At the end of month 2, it is essential to have a level of C+ support or above with Edelgard. To do this, you can simply think of increasing your affinity by talking, having tea or offering various gifts at your disposal.
3 - Follow Edelgard during the battle in the tomb of the goddess:
During the battle in the tomb of the goddess, Rhea asks you to run Edelgard. You have two options: "I must execute Edelgard" or, if you have performed the above actions correctly, "I must protect Edelgard". Protect the latter and you will then unlock the 4th scenario of the game!