The story begins with Chrom, a member of the Hero's party, receiving a surprise marriage proposal from his lover, the priestess Irina, after the war against the Demon Lord ends. Chrom accepts, solidifying their future together. The next day, the party attacks a frontline Demon Lord army base, facing a powerful Dark Dragon. The party, consisting of Chrom (a mage), Irina (a priestess), Valery (a sage and Chrom's mentor), Riot (a warrior), Fara (a swordswoman), and Margo (a knight), works together to defeat the dragon. The Hero, Yuno, a young man with golden armor and a kind demeanor, delivers the final blow with his holy sword, Weiss. Chrom considers Yuno not just a hero but a close friend.
Three years prior, the Demon Lord had revived and conquered half the world. Seven Heroes were chosen to combat him, each forming a party. Chrom's party has been fighting for five years, and they are now preparing to face Franjuras, a Demon Lord executive. After defeating the Dark Dragon, Chrom heads upstairs to his party members' rooms and witnesses Irina secretly entering Yuno's room. He initially dismisses it but, driven by suspicion, overhears Irina confessing her love for Yuno and engaging in intimate acts with him. Devastated, Chrom spends the night unable to sleep.
The following morning, the party, including Yuno and Irina, act as if nothing happened, their cheerful demeanor contrasting with Chrom's inner turmoil. Valery proposes they attack Franjuras, and Yuno agrees, noting their party's lagging success compared to others. As they venture into a forest towards Franjuras's castle, Chrom is suddenly overcome by intense pain and collapses. Valery reveals this is a ritual, and Irina coldly informs Chrom he is to be a sacrifice to empower Yuno. Riot, Fara, and Margo also turn on him, confirming it was a collective decision. Valery casts a forbidden spell, "Chains of Darkness," draining Chrom's life force and magic, turning his hair silver and his limbs skeletal. He realizes he has been betrayed and used as a tool.
Chrom awakens in the forest, his magic gone and his body severely weakened, bound by dark chains. He learns from a voice within him, the embodiment of his awakened "Darkness," that his sacrifice was meant to fuel Yuno's "Light." The Darkness grants him new skills, including "Fixed Damage," which deals a small amount of damage to enemies within a radius. He is then attacked by a pack of Sword Wolves, but his new skills, though initially weak, activate and kill them. The Darkness explains that his negative emotions will strengthen its power.
Chrom, now a shadow of his former self, struggles with his weakened body and the loss of his magic, the very thing he dedicated his life to for Irina. He encounters bandits who mock his frail state but are quickly dispatched by his growing "Fixed Damage" skill. He realizes his power grows with his hatred and despair. He vows revenge on Yuno, Irina, Valery, Riot, Fara, and Margo.
Two years later, Chrom has honed his "Fixed Damage" skill to an incredible level, capable of instantly killing any foe within a 10-meter radius. He encounters a young knight, Shya Flamlight, who is being attacked by soldiers serving Duke Riot. Chrom intervenes, effortlessly dispatching the soldiers with his skill. Shya reveals her sister was tortured and murdered by Riot, and she seeks revenge. Chrom agrees to help her, as Riot is also a target of his vengeance.
Chrom and Shya infiltrate Riot's territory. Riot, now a decadent Duke, is confronted by Chrom, who reveals himself. Riot, protected by a powerful artifact, the "Seal Crest," initially dismisses Chrom. However, Chrom's skill bypasses the artifact's defenses, dealing immense damage. Riot, stripped of his power and pride, is slowly tormented by Chrom's approach, his fear and despair growing with each step Chrom takes. Chrom recounts his betrayal and the loss of his cherished companions, finally delivering the killing blow to Riot, marking the first step in his revenge.
Chrom and Shya visit the grave of Shya's sister, Leeah. Shya pledges to accompany Chrom on his quest for revenge against the remaining five members of the Hero's party: Yuno, Irina, Valery, Fara, and Margo. Chrom, though initially hesitant, accepts her offer, recognizing her unwavering resolve. Meanwhile, Yuno and Irina, now engaged and celebrated as heroes for defeating the Demon Lord, learn of Riot's death and express concern about their own safety, Yuno vowing to protect Irina with his holy sword.
