Here's how such an AU, its arcs, and its core elements might work:
The Core Premise: The Nomu King/Queen
The Mastermind: Dr. Kyudai Garaki (or a similar brilliant but deranged bio-engineer) is the sole architect of the Nomu threat. In this AU, he never truly served All For One, or perhaps he inherited AFO's research and took it in an even more extreme direction. He has experimented on himself, integrating multiple Quirks and undergoing a partial "Nomu-fication," becoming a grotesque but highly intelligent hybrid. He sees himself as the "Nomu King/Queen" – the vanguard of a new, superior species.
Motivation: Garaki genuinely believes that Quirks have led humanity to a dead end, creating a chaotic, inefficient society. He views Nomus as the ultimate, perfected beings: devoid of ego, perfectly obedient, endlessly adaptable, and capable of holding multiple Quirks without mental strain. His goal is not to rule through fear, but to "evolve" humanity, forcibly transforming people into Nomus to create a "harmonious" and "efficient" new world.
Absence of LoV/AFO: The League of Villains never forms, or they are merely small-time criminals easily crushed by the emerging Nomu threat. All For One might be a mythical figure from the past, his research notes merely a stepping stone for Garaki, or he might not exist at all, making Garaki the undisputed scientific evil.
Impact on Hero Society
Shift in Focus: Heroes can't simply "arrest" Nomus. The fight becomes about containment, incapacitation, and ultimately, destruction of their vital point (the brain). The emphasis shifts from individual villain capture to large-scale defensive operations and understanding Nomu biology.
Public Fear: The terror isn't just about destruction; it's the existential dread of losing one's self, of being transformed into a mindless monster. Body horror is a constant theme.
Quirk Research: Hero society's scientists and support departments become paramount, desperately racing to understand Nomu anatomy, find weaknesses, or even develop a "cure" or counter-Quirk.
AU Arcs & Story Progression
Arc 1: The Silent Invasion & First Roar (U.S.J. Equivalent)
The Setup: U.A. High is still the premier hero academy. Midoriya, Bakugo, and the rest of Class 1-A are eager students. However, there's an underlying tension in society – whispers of "missing persons" and strange, unidentifiable attacks.
The Event: Not a direct attack on U.S.J. by a small group, but a sudden, coordinated emergence of Lower-Tier and Middle-Tier Nomus in multiple urban centers simultaneously. These are the "foot soldiers" – strong, but still somewhat predictable. The U.S.J. training exercise might be interrupted by a few stray Nomus, giving Class 1-A their first terrifying taste of the threat.
Hero Response: Mass chaos. Pro Heroes are overwhelmed. Their usual tactics of restraint and capture fail against regenerating, mindless foes. All Might, the Symbol of Peace, is forced to push past his limits, realizing that these creatures are different.
The Revelation: Amidst the destruction, a chilling broadcast cuts through all channels: Dr. Garaki, in his partially Nomu-fied form, appears on screen. He declares the "Dawn of the Nomu Age," proclaiming humanity's obsolescence and the Nomus as the "next step." He demonstrates a terrifying new ability – perhaps controlling a large number of Nomus with a thought, or even a localized transformation Quirk that turns a few unlucky civilians into proto-Nomus. He then vanishes, leaving behind a trail of destruction and an unshakeable fear.
Arc 2: The Hunt for the Source & The Intelligent Threat (Hosu/Pro Hero Equivalent)
The Investigation: Following the initial attacks, hero society scrambles. Detective Tsukauchi and top pro heroes like Endeavor and Hawks lead an investigation into the Nomu's origins. They find clues pointing to hidden, highly advanced laboratories.
The First High-End: During this investigation, a new class of Nomu emerges: the High-Ends. These aren't just mindless brutes; they retain fragments of their past personalities, are capable of rudimentary speech, and exhibit terrifying strategic intelligence. A "Hood"-like Nomu might be unleashed, not just to fight, but to observe and learn the heroes' tactics, taunting them about their "inferiority."
Heroic Adaptation: Heroes begin to develop new strategies: targeted strikes to the brain, coordinated attacks to overwhelm regeneration, and the use of Quirks that can disable or destroy on a cellular level. Eraser Head becomes invaluable for temporarily neutralizing their Quirks.
Midoriya's Role: Midoriya, with his analytical mind, becomes crucial in understanding Nomu weaknesses and developing counter-strategies. He might begin to see the tragic side of the Nomus, realizing they were once human.
Arc 3: The Factories of Flesh & The Horrific Truth (Hideout Raid Equivalent)
The Discovery: Heroes pinpoint Garaki's primary Nomu production facilities. This isn't a "hideout raid" to rescue one person, but a desperate attempt to shut down the source of the monsters and prevent further "evolution."
The Raid: Multiple hero teams, led by the top pros, assault the labs. They face overwhelming numbers of all Nomu tiers, including specialized defensive Nomus. The labs are a grotesque sight: vats of mutating flesh, half-formed Nomus, and chilling evidence of human experimentation.
The Ethical Dilemma: Heroes encounter individuals in various stages of transformation, forcing them to confront the horrific reality: Nomus were once people. This raises profound ethical questions about what constitutes "life" and whether these creatures can be saved.
Garaki's Ideology Revealed: Garaki, perhaps cornered, reveals his full, terrifying vision. He might showcase his own perfected hybrid form, demonstrating what he believes is humanity's glorious future. He explains the process, chillingly detailing how he "improves" on human biology.
Arc 4: The Nomu War (Paranormal Liberation War Equivalent)
The Full-Scale Assault: Enraged by the heroes' interference, Garaki unleashes his full might. A massive army of Near High-Ends and a select few perfected High-Ends are deployed across Japan, targeting major cities, hero agencies, and even U.A. High. This is a war of attrition, a true "Nomu Apocalypse."
Global Response: The threat is so immense that international heroes might arrive, forming a united front against the Nomu hordes.
Key Battles: Iconic heroes face off against intelligent High-End lieutenants, each with unique Quirk combinations and strategic minds. These battles are brutal, pushing heroes to their absolute limits, with heavy casualties.
The "Evolution" Event: Garaki initiates his ultimate plan: a widespread, forced "evolution" event, perhaps through a massive Quirk activation or a specialized Nomu that disperses a transformation agent. The goal is to convert a significant portion of the population into Nomus, making the process irreversible.
Arc 5: The Symbol's Last Stand & The Aftermath
The Final Confrontation: All Might (or Midoriya, having fully inherited One For All and become the new Symbol of Peace) confronts the "Nomu King/Queen" in a climactic, desperate battle. This isn't just a physical fight; it's a clash of ideologies: humanity's individuality and free will versus Garaki's vision of forced, mindless perfection. The fight might involve exploiting Garaki's own Nomu-hybrid weaknesses.
The Defeat: Garaki is finally defeated, but the victory is bittersweet. The "evolution" event might have succeeded in part, leaving a portion of the population transformed, or the sheer devastation caused by the Nomu War leaves society on the brink.
The Aftermath:
Rebuilding: Society faces the monumental task of rebuilding.
The "Cure": Scientists race to find a way to reverse the Nomu transformation, or at least stabilize those partially affected. This could be a new ongoing challenge for the series.
Ethical Questions: The existence of Nomus forces society to re-evaluate what it means to be human, the ethics of Quirk research, and the responsibility of power.
Lingering Fear: The memory of the Nomu War and the threat of forced evolution would forever shape the world, leading to stricter regulations on Quirks and bio-engineering.
This AU would offer a constant sense of dread and a different kind of heroism, focusing on survival, scientific ingenuity, and the fight to preserve humanity's very essence against a truly monstrous, yet tragically created, threat.