This is the timeskip event. Byleth will wake up after five years since the last battle, right in time on the day of the promised date of reunion of your class. After a few cutscenes, you’ll be immediately taken to a battle with Claude.
Don’t worry about being only the two of you against a bunch of enemies. Claude will start off as a powerful Wyvern Master now, along with his unique wyvern Battalion. Just stick close together and lure the enemies towards your position instead.
After a couple of turns, Lorenz, Ignatz, Hilda, and Leonie will arrive from the northwest and southwest corners of the Map respectively as reinforcements. Regroup or have them proceed carefully as they engage and take out more enemies. It’s recommended to have Byleth head to the north since there’s a lot of locked Chests and doors that he can easily open by getting extra door and chest keys via the Convoy menu.
On the following turn, Marianne, Lysithea, and Raphael will arrive on the northeast corner of the Map. You can have them attack the enemies there; just make sure they don’t get isolated or focused by enemy archers.
With your team complete, you can talk to your students in battle to increase Support or you can have them approach you to talk to you instead. Continue moving your main group to the north and clearing the enemies there. You can avoid the central position where the bandit leader and his guards are staying as you open the Chests and doors. Open all chests since they contain really good loot, as shown below. If you don’t have enough chest keys, you can always send your units (especially Fliers like Hilda and Claude) to Byleth’s position and have them access the Convoy menu to add the keys to their inventory.
After looting the Chests and defeating all other enemies, you can now engage the bandit leader and his men. Make sure to divide your forces and have at least one or two units guarding the gates above and below his position. This is because after defeating him, he will respawn with a handful of his men and will attempt to escape via the escape points to the northwest and southwest corner of the Map. You can easily cut off their escape path by having units guarding their vicinity around the said gates. Finish off the remaining enemies to complete the mission.
After the battle and reuniting your students, you’ll be back in the ruins of the monastery, planning your next course of action. In this route, your allies from the Church will join your army once again. After a few more cutscenes, the chapter will be over.