|Gunter||Great Knight||Automatically after reaching the northernmost part of the map.|
This map has a new gimmick you likely haven't seen yet: It's dark and you can't see anything. In order to move forward, you must move to the end of the tunnels, revealing a new room that is swarming with enemies. Or, if you move to the end of the wrong tunnel, you are warped right back to the center of the room. Lead with Corrin, singing with Azura and healing/adjacent pairing with Jakob or Felicia. The area you want to Seize is directly northwards, but as you can see, the map is much more expansive than that.
Heading westwards reveals a slew of chests, so if you want all the loot you will have to slow down and take your time. You'll get more experience out of it too. Most of the enemies near the chests will be holding Chest Keys, since you don't have access to Locktouch right now. Use the forts in the center of the room to rest and recover if necessary (preferably use Felicia or Jakob so they can get experience).
So continue to work your way forward in this fashion.
To the north from here we get a surprise: Gunter is alive! Alive and well even, ready to help us out. He will become controllable right away. He is much stronger than any of the enemies here, and will be an experience sink, so don't use him to defeat your foes. Instead, pair him up with the Avatar in defensive stance to boost your Avatar's stats. Then you can approach the boss.
The boss is a Berserker, he won't move to attack you, so gang up on him as you need to, refresh your Avatar with Azura and heal with Jakob or Felicia. Berserkers have a pretty high critical hit chance but this guy is at a low level with a weak weapon so he shouldn't be much to worry about. If you're playing on Normal difficulty, feel free to Battle Save.
Once he falls, he will reveal the tile that you need to Seize. Using any unit in your (currently very tiny) army, walk onto the tile, then from the menu of actions you can perform, select the "Seize" command. This will end the battle here and initiate another series of cutscenes. Back up to Hoshido we go! Do what you need to do in MyCastle before proceeding onwards.
Note: Paralogue 1, "A Tragic Start", which allows you to recruit Mozu, will be unlocked after this chapter. You should probably do it as soon as possible so that Mozu won't be resource-intensive to level up, as she is a very powerful unit given some TLC. The Paralogue itself will also be a good chance to grab some experience! Check the Paralogues section of this guide for all the Paralogue walkthroughs.