In a fantasy world where five humanoid races are confined to a single continent surrounded by vast territories ruled by dark beasts, Clevatess stands as one of the Four Lords of Dark Beasts, a being of immense power with a mane as bright as light and fur as dark as night. After a thousand years of peace, he is disturbed by a group of thirteen heroes sent from the Kingdom of Hiden on a suicidal mission to slay him. Clevatess annihilates the heroes with effortless brutality and, questioning why such fragile humans would dare attack him, launches a retaliatory assault on Hiden's capital, laying waste to the kingdom. Amid the ruins, a dying woman begs him to protect a newborn baby, vowing that if allowed to grow, the child will prove humanity's worth. Moved by curiosity rather than compassion, Clevatess agrees. He revives one of the slain heroes, a young warrior woman named Alicia, to assist in caring for the infant, whom he names Luna. Thus begins an unlikely journey as the beast king, the undead hero, and the orphaned child wander the land together. Clevatess seeks to understand humanity and decide whether the race deserves to survive or be exterminated, while Alicia struggles with her servitude to the monster who killed her and her comrades.

The main characters include Clevatess, the titular King of Dark Beasts, who initially views humans as insignificant insects but develops a protective instinct toward his adopted family. Alicia, known as the Corpse Hero or Undead Hero, is a skilled swordswoman and the daughter of the legendary Swordmaster Margo Glenfall. Resurrected and bound to Clevatess through his dark ichor, she possesses enhanced resilience and a form of immortality, though her body bears the scars of her death and she must grapple with her new existence as an unwilling servant. Luna is the infant, the last heir of the fallen Hiden kingdom, whose upbringing becomes central to the fate of humanity. Later, the party acquires Nell, a slave who becomes Luna's wet nurse, and after her death, she is revived by Clevatess and renamed Nellel.

The narrative follows the trio's perilous journey across the continent of Edthea, navigating threats from bandits, rival nations, and the machinations of other dark beast kings. Clevatess adopts a human form to move among humanity undetected. One notable early arc involves a confrontation with the Ravens bandit gang, during which Alicia is severely injured and thrown into a mine shaft. There, Clevatess appears to her in a small form made of his ichor, explaining that her regeneration depends on him. He orders her to destroy the bandits to free a potential wet nurse named Nell without revealing his identity, leading to a desperate battle where Alicia deliberately collapses scaffolding and jumps into water infested with dark beasts to retrieve a legendary Regalia weapon. Another arc follows their capture by slavers and eventual escape, further solidifying the found family dynamic as they protect Luna from those who would exploit or destroy him. The story explores themes of interspecies coexistence, redemption, and whether a child raised by monsters can become the salvation of a dying world.