“After Roy's defeat of Zephiel, Brunya, and finally the dragon Jahn, Roy's army goes to fight Idunn in the Dragon Temple to end Zephiel's crazed attempt to have her make a "perfect world" of War Dragons.”
Beyond the Darkness (暗闇の向こう Kurayami no mukō) is the Final Chapter of Fire Emblem: The Binding Blade.
Idunn and two manaketes are on screen at the start of the battle, and two powerful Manaketes arrive as reinforcements at the start of each turn. The chapter ends when Idunn is defeated. If Roy strikes the final blow using the Binding Blade, a different, happier ending will be shown in which Idunn breaks free of the spell she was under and returns to Arcadia with Fae and Sophia. This chapter is unlocked by defeating the king of Bern and having all of the divine weapons intact.
Note: When you begin this chapter, you will not be able to change your party and restock. So do not be careless in your items and party members in Chapter 24.