When Soma and his companions cross paths with Aina on her way to visit her father—the Dark Lord—they decide to accompany her to the castle that lies deep in the forbidding mountains of the land of the devils. There, Soma discovers that the mystery-shrouded Dark Lord and the renowned hero Sir Iori are the same person: Iori Kanzaki, Soma's old friend from a past life. The reincarnated friends begin to put the pieces together: how the devils came to be, what happened between their nations, and why the two have been brought together once more. But as they do, a plot is brewing among the former Dark Lord's faction, soon to reach a boiling point.