Chapters 3-12 of the light novel detail the protagonist, Allen, a highly ranked hero, grappling with the disillusionment of his profession and a subsequent betrayal, leading him to an unprecedented alliance.
The narrative begins by establishing the traditional dynamic between heroes and demon lords as destined enemies locked in eternal conflict for the freedom of humanity versus the dominion of the demon lord. However, it quickly introduces a modern interpretation where this dichotomy is less absolute.
Allen, the "Strongest Hero" and ranked number one, finds himself in a dire situation not of combat, but of existential crisis. The beautiful Demon Lord Lilith offers him a lucrative employment contract, appealing to his dissatisfaction with his current life. This offer deeply unsettles Allen, whose recent experiences have fueled his discontent. Ten days prior, after a perilous battle against the S-rank Demon Lord Lucifugus, Allen was rewarded with a meager sum of ten thousand yen, barely enough for half a month's living expenses. The king justified this by deducting costs for collateral damage, despite Lucifugus being the primary cause. Allen's protests were met with stern reprimands, emphasizing his duty as humanity's hope.
Meanwhile, unseen by Allen, the king and his ministers plot his demise. They view him as a potential threat, too powerful to control, and plan to eliminate him, framing it as a heroic sacrifice.
Returning to his room, Allen laments his meager earnings and the lack of rest. His dedicated maid, Sarah, informs him of his next assignment: an F-rank demon lord. This is highly unusual, as such low-ranked demons are typically handled by novice heroes. Allen, suspecting foul play, accepts the order.
His journey to the remote demon lord's castle is uneventful, with no encounters, which is highly irregular. Upon entering the castle, he finds it eerily empty. He eventually confronts Lilith, who reveals she summoned him not for battle, but for recruitment. She offers him a position with excellent benefits: a fixed salary, bonuses, and ample paid leave, a stark contrast to the hero's exploitative conditions. Allen, though tempted, initially refuses, citing the inherent enmity between heroes and demon lords. However, Lilith, possessing the "Judgment's Blessing" (which allows her to discern truth), reveals she knows of his dissatisfaction with his working conditions and meager pay. She argues that his current path leads to burnout and offers him a chance at a better life, promising coexistence for all races, not just humans and demons.
Allen's "Judgment's Blessing" confirms her sincerity. Just as he is about to accept, three other heroes, sent by the king, arrive. They reveal their true mission: to assassinate Allen, whom the king deems a threat. They attack, and despite Allen's immense power and rapid healing, he is mortally wounded by a surprise attack from one of the heroes. As he lies dying, Lilith, empowered by a pendant from her father, the Great Demon Lord Satan, transforms into her adult form and breaks free from the mind-controlling Holy Sword "Lovers" wielded by the treacherous hero Sixxuz.
Realizing the kingdom's betrayal and the hypocrisy of the heroes, Allen, now free from his heroic obligations, accepts Lilith's offer. He declares himself no longer a hero but a "demon lord's sword," vowing to protect her and pursue her dream of coexistence. He defeats Sixxuz, disabling his Holy Sword and sending him flying, effectively ending his threat.
The chapters conclude with Allen and Lilith beginning their new partnership, with Allen agreeing to her terms and preparing to investigate her situation further, including the whereabouts of her few remaining subordinates. The narrative sets the stage for an unprecedented alliance between a former hero and a demon lord, promising significant upheaval in the world.
