Iago discovers that Hinoka is still alive, so the Nohrian siblings must fight him and Hans.
Send high defense units west and high resistance units east. Once you reach the south part of the map, Hans will attack. If you kill him, you get the S-rank axe Aurgelmir.
Once you have dealt with Hans and the Generals, go north and open all the chests, then kill Iago, and finally Seize the tile to end the battle.