After defeating Garon, you must defeat the possessed Takumi.
Defeat the Boss
Takumi is in the south part of the map, paired up with a replica of himself. At various times throughout the battle, he will charge up a special attack that will damage all of your units. However, there are Dragon Veins throughout the map that will put up barriers that block MOST of Takumi's attack. The left and right edges of the corridor will still get hit, so move your units away from the sides if you see Takumi charging up an attack.
Be sure to check Takumi on each turn to find out if he's charging up, and DON'T use the Dragon Veins if he is not charging up.
As you make your way south, enemies and reinforcements will appear from the corrupted areas on the left and right. You can't walk into those areas, but the enemies can walk to the central corridor from those areas.
Faceless will eventually start appearing from the corrupted areas.
When you finally get close to Takumi, wait for him to use a charged attack, then send your strongest units to fight him. You won't need to defeat his Replica. The battle will end when you defeat the Takumi in front. Congratulations! After the credits, when it says The End, press A and you will be prompted to turn five of your units into Einherjar units, which you can purchase from the Einherjar Shop in your castle (if you build it), allowing you to use those units in battle.