The Demon Sword Master of Excalibur Academy, Vol. 13 (light novel)

DARK LORD BATTLESHIP ENDYMION, FULL SPEED AHEAD! Leonis and his army are headed for the Otherwordly Castle to confront the Undead King. On their way to the Void World, they fight alongside the Dragon Lord, Veira, dispatching hordes of Void monsters. Meanwhile, now that Elfiné has been rescued by Riselia, it's time for the two to make their daring escape. However, things may not go so smoothly as they become trapped underground in the control tower. The situation goes from bad to worse as the messenger of the Apostles, Nefakess, blocks their way in his true, monstrous form. It's the Dark Lord alliance versus the Undead King's men! Who will come out on top?