In a near-future Tokyo reimagined as a demonic metropolis, the human world and the netherworld have merged following a catastrophic event known as the Great Integration. The city is now a vertical labyrinth of neon-lit skyscrapers, subterranean slums, and floating fortresses, ruled by demon lords who have divided the urban sprawl into feudal territories. Humans coexist with demons, but only as second-class citizens, their souls and labor exploited to fuel the city’s occult infrastructure. The central conflict ignites when a mysterious human youth awakens in a derelict ward with no memories, only to discover he possesses a rare and forbidden ability: the power to nullify demonic magic and absorb the life force of any demon he kills. This power makes him the prophesied “King of Ruin,” a being destined to either overthrow the demon lords or become the most tyrannical ruler the city has ever seen.

The protagonist, Ren, is a quiet and introspective young man who initially seeks only to recover his past and survive the city’s brutal underworld. He is soon taken in by a rogue human resistance cell called the Dawn Pact, led by the sharp-tongued and pragmatic tactician Saya. Saya sees Ren as a weapon to win the underground war against the demon lords, but she is wary of his growing power and its corrupting influence. Ren’s closest ally is a defected demon noble named Mephisto, a flamboyant and cynical information broker who was once a high-ranking vassal to one of the ruling archdemons. Mephisto assists Ren out of a self-serving desire to see the current hierarchy crumble, yet he constantly warns Ren that the nullification ability is a double-edged sword, as each use feeds a dark hunger within him that threatens to erase his humanity.

The narrative unfolds through several major arcs. The first arc, the Slum Ward Uprising, sees Ren and the Dawn Pact fighting to liberate a human ghetto from a cruel overseer demon, forcing Ren to confront his own thirst for revenge and the monstrous glee he feels when draining a demon’s power. This leads to the Court of Whispers arc, where Ren infiltrates a neutral zone of neutral demon merchants and aristocrats, learning that the Great Integration was not an accident but a ritual orchestrated by a cabal of ancient demon sages to summon a new god of chaos. The third major arc, the Succession War, erupts when the death of a powerful archdemon throws the city into a violent power struggle, and Ren is manipulated by multiple factions—including a fanatical human purist group and a reformist demon queen—to claim the throne. Throughout these events, Ren uncovers fragments of his forgotten past, revealing that he was not born human but was originally a demon general who chose to be reincarnated as a mortal to escape his own genocidal duties. This twist forces Ren to reconcile his current identity with his former self, as the city’s fate hinges on whether he will embrace his demonic heritage to rule as a tyrant or use his nullification power to permanently sever the connection between the two worlds, which would annihilate both races in the process. The series culminates in a final confrontation atop the Celestial Spire, where Ren must choose between Saya’s vision of human liberation, Mephisto’s chaotic neutrality, and the seductive call of his original demonic nature, leading to a resolution that reshapes the very laws of magic and reality within the demon city.