After escaping the battlefield, Azura brings you to a mysterious place and reveals some of her secrets.
The objective is to reach the seize tile, but as you can see from the map, there doesn't seem to be a path to it. However, there are a couple of dark hallways, and it turns out that if you move your unit to the end of a dark hallway, a new path will be revealed.
From where you start, send a unit to the end of the west hallway. (The north one leads to a dead end.) When the path is revealed, some enemies will appear out of a fort. Defeat these, and rest up at the fort if you need to.
From here, there are two ways to go, but it's a bit better to go to the left. So send a unit to the end of the west hallway, and you will find another fort with enemies. Kill them, then go west again.
Now there will be more enemies, but also a couple of treasure chests. Defeat the enemies in this area to get some Chest Keys that you can use to open the chests. (Just stand on one of the four tiles adjacent to the chest, then choose Chest to open it.)
From here, go north. There will be more enemies and another chest. Keep going north, fighting more enemies, until you can't go north any farther. From there, go east and you will find none other than Gunter. He joins you. Be sure to have him fight some enemies so he can level up. But be aware that your units can't improve their relationship with him right now (i.e. he can't have Support conversations with anybody at this point).
At this point, you could go east to reach the enemies guarding the seize tile, but I recommend going south instead, because there is another treasure chest down there, and if you go east from that chest, you find another chest. In addition, there are a few more enemies to fight if you go west from that second chest.
When you are ready, go east from where you found Gunter, and you will reach the final cluster of enemies in this map. The unit guarding the Seize tile is a Berserker who wields a Steel Axe. He is weak to swords, so Corrin should attack him. After that, stand on the seize tile and choose Seize if you are ready to end the battle.