In this alternate "what if" scenario, let's assume Tomura Shigaraki survives the Final War—perhaps by narrowly escaping death after a brutal clash with Deku, or even "winning" in a twisted sense by shattering hero society enough to create a world of chaos where quirks like his aren't vilified but embraced as tools of revolution. The heroes are scattered, All For One's influence lingers in Tomura's fragmented mind, and Japan is a fractured landscape of ruins and opportunistic villains. Now, onto how he might cross paths with young Koki Terumoto and take him under his wing in a dark mentorship. I'll weave this as a narrative outline, staying true to the characters' essences while exploring the mentor-apprentice dynamic.

The Encounter

Shortly after Koki's quirk manifests in his family's living room—that ominous dark sludge spreading like an inescapable void, swallowing furniture and leaving his relatives in horrified awe—the Terumoto family reacts with the same fear and cruelty as in canon. The elder demands his death or imprisonment, labeling him a "freak variant" that could tarnish their reputation. But in this timeline, the family doesn't get the chance to fully enact their plan. The Final War's chaos spills into civilian life earlier; riots and quirk-fueled skirmishes turn neighborhoods into battlegrounds.

As the family debates Koki's fate in their basement, a massive decay wave from a distant villain clash (orchestrated by Tomura's forces) crumbles part of their home. The structure collapses, killing most of the Terumotos in the debris. Young Koki, barely five or six, survives by instinctually unleashing more of his Darkness quirk—creating a protective sinkhole that engulfs the falling rubble around him, though it leaves him trapped and terrified in a pocket of shadow.

Tomura, scavenging the ruins for resources or potential recruits amid his survival, senses the anomaly. His own quirk, Decay, resonates with the destructive essence of Koki's power; it's like a echo of erasure, but inverted—where Tomura disintegrates, Koki engulfs and hides away. Drawn by curiosity (and perhaps a subconscious recognition of his own abused childhood under All For One), Tomura decays his way into the wreckage. He finds the boy huddled in a cocoon of writhing black sludge, eyes wide with fear, mouth already bruised from his family's initial attempts to silence him.

"You're not broken," Tomura rasps, his voice a gravelly whisper through his scarred lips. He doesn't touch the boy at first—too risky with his quirk—but extends a gloved hand, pulling him free. Koki, starved for any kindness, doesn't resist. Tomura sees a mirror of his past self: a child discarded for a power society deems monstrous. In his mind, this isn't pity; it's opportunity. "The world hates us because we're the ones who can end it. Come with me, kid. I'll show you how to make it pay."

The Mentorship Begins

Tomura doesn't "adopt" Koki in a nurturing sense—his version of mentorship is raw, survivalist, and ideological. They hole up in abandoned League safehouses, moving constantly to evade remnant heroes like Deku or Hawks. Tomura, still healing from his wounds (maybe with vestiges of All For One's quirks aiding regeneration), treats Koki as a protégé rather than a son. It's a twisted echo of how All For One molded him, but with Tomura's personal vendetta against hero society infused in every lesson.

Quirk Training: Tomura pushes Koki to master Darkness without mercy. Early sessions are brutal: "Don't hold back. Let it consume." Koki learns to shape the sludge into tendrils for grabbing enemies, pits for trapping foes, or even veils for stealth. Tomura demonstrates synergy—using Decay to weaken structures, then having Koki's Darkness swallow the remains whole. Drawbacks emerge: overusing Darkness drains Koki's stamina, causing the sludge to backfire and "sink" parts of his own body temporarily, leading to numbness or temporary paralysis. Tomura teaches him control through pain, echoing his own upbringing: "Pain means you're alive. Use it." By age 10, Koki can cover entire city blocks in shadow, creating "void zones" where heroes vanish without a trace.

Ideology Indoctrination: Tomura fills Koki's head with anti-hero rhetoric. "Your family threw you away because you weren't 'normal.' Heroes do the same to everyone who doesn't fit their pretty world." He shares stories of his own abuse (as Tenko Shimura), forging a bond through shared trauma. Koki, who in canon grows bitter at the world's joy while he suffered, here channels that rage earlier. Tomura molds it into purpose: "We're not villains. We're the reset button." Koki idolizes him, calling him "Sensei" in quiet moments, though Tomura brushes it off with a smirk.

Daily Life and Bonds: It's not all darkness. Tomura, in rare softer moments (influenced by lingering memories of the League), teaches practical skills—scavenging food, hacking basic tech (with Spinner's old gear), or even playing crude video games on salvaged consoles to "strategize." Koki becomes Tomura's shadow, literally and figuratively, using his quirk to hide them during raids. Over years, a dysfunctional loyalty forms; Koki sees Tomura as the father figure he never had, while Tomura views him as a weapon and successor. If the League remnants (like Dabi or Toga) survive, they form a ragtag "family"—Toga doting on Koki like a little brother, Dabi teaching him fire synergy with Darkness for smokescreens.

Long-Term Outcomes

Eight years post-war (aligning with canon's timeskip), this world is no utopia. Tomura's "victory" means fragmented territories ruled by quirk overlords, with heroes operating underground. Koki, now a teenager, is a formidable villain-in-training:

Strength: His quirk evolves under Tomura's guidance. He can now "store" objects or people in pocket dimensions within the sludge, retrieving them later (a la Kurogiri's Warp Gate, but void-based). Combined with Tomura's tactics, he's a nightmare for resistance fighters—sinking entire squads into oblivion.

Conflicts: Koki grapples with morality. Encounters with survivors (like that old housewife from canon) plant seeds of doubt: "Why destroy everything when some people are kind?" Tomura quashes it: "Kindness is a lie. They only help when it's easy." This could lead to internal strife—perhaps Koki betrays Tomura one day, or they clash in a mentor-vs-apprentice showdown, echoing Tomura's own rebellion against All For One.

Epilogue Tie-In: If we twist canon's epilogue, Koki doesn't end up at U.A. Instead, he infiltrates it as a mole under Tomura's orders, posing as a reformed quirk user. There, he meets Kota or Deku (now a teacher), creating tension between his indoctrinated villainy and glimpses of heroism. Or, in a darker end, Koki helps Tomura launch a final assault, using Darkness to engulf U.A. entirely.