When the blood of the magic swordsman Sparta spread to a planet without the hero Claude, the trajectory of fate was completely rewritten.

Kain, Vergil's descendant left behind in another world, was adopted by a kind woman from Nibelheim and formed a childhood sweetheart with Tifa. However, demon blood is both a gift and a curse. To protect his beloved, he awakened in flames, only to fall prey to Shinra's inhumane experiments.

Inheriting the legacy and destructive sword of the hero Zack, he arrives in Midgar with his memory clouded. Here, he connects with the soul of the destined flower girl Alice, befriends the lively ninja Yuffie, and even attracts the morbid yet passionate love of Queen Scarlet of Shinra.

With his mortal body, he rewrites tragedy time and again; with the power of a demon, he strikes back against fate. From Midgar to the Great Hollow, he slashes through despair and protects every bit of warmth around him. When the wings of light of the True Demon unfold, he will wield the ultimate secret, the fusion of all bonds, and personally "slay the gods."

This is the story of a lost demon boy who searches for a home in the world of Final Fantasy, protects his family, and ultimately becomes a legend. However, when the dimension-severing blade, [Yanma Blade], awakens within his soul, he and his family face an even more magnificent, world-spanning epic.