Chapters 3 through 12 of the light novel detail the immediate aftermath of the Demon Lord's defeat and the subsequent framing and exile of the "Supreme Sage" Mathis Lutius, who is revealed to be the protagonist, Sieg Andreas, in disguise.
The narrative begins with Sieg, a blonde-haired, blue-eyed warrior, impaling the Demon Lord with a holy sword, declaring his friend Mathis as the true hero behind the victory. They escape the collapsing cavern using ancient teleportation magic, witnessing the catastrophic destruction of the Demon Lord's power source, the "Netherworld's Magic Furnace." Mathis, a black-haired, black-eyed scholar, expresses concern over Sieg's excessive use of magic to protect him, but Sieg asserts that Mathis, as the creator of heroes, is more valuable than any single warrior. Their close bond is evident, with Sieg deeply respecting Mathis's intellect and attributing his own success to his friend's guidance.
They are met by their teacher, Isabella Charrier, a young and capable strategist. After a brief reunion, Isabella directs them to General Gadif's forces. However, upon reaching the capital, news of Sieg's death in a demon attack spreads, followed by Mathis being accused of treason and complicity in the attack.
The story then flashes back twenty years, recounting the rise of the demon forces fueled by the Netherworld's Magic Furnace and the subsequent emergence of Sieg and Mathis. Their combined efforts led to the Demon Lord's defeat and the furnace's destruction, bringing peace. However, this peace was shattered by Sieg's alleged death.
The trial of Mathis Lutius unfolds in the Grand Cathedral of the Holy Capital. Mathis is accused of aiding demons and murdering Sieg. Despite Isabella's staunch defense, claiming their bond was deeper than family, and the testimonies of those who benefited from Mathis's strategies, the court, influenced by public opinion and veiled accusations of jealousy and fear of Sieg's power, sentences Mathis to exile with a "Watcher's Collar." It becomes clear that Mathis is being framed by powerful figures within the Holy Kingdom who feared Sieg's immense power.
The final chapters reveal that the "Mathis Lutius" on trial is actually Sieg Andreas in disguise. He has assumed Mathis's identity to clear his friend's name and uncover the conspiracy. He travels to a remote village where the exiled Mathis was sent. There, he lives as Mathis, using his friend's research and his own formidable abilities, including advanced magic and a unique ability to communicate with monsters, to investigate the conspiracy. He encounters Luna, a village girl with exceptional talent, and Hakeem, a disgraced nobleman who once refused to testify against Mathis. Sieg uses his powers to protect Luna from a dragon and extract information from the assassins sent by Duke Davon, the presiding judge at Mathis's trial. The chapters end with Sieg preparing to return to the capital, determined to expose the truth and restore Mathis's honor.
