League of Villains: Undercover

Nine watches a storm tear a park apart from an alley nearby and finds it soothing, convinced everything around him is destined to dissolve like mud in the rain. Some time afterward he seeks out Doctor Daruma Ujiko of the League of Villains. The doctor observes that although Nine's Quirk has been eating away at his own cellular tissue, he has passed through life without anyone realizing anything was wrong with him, and it is precisely that combination that makes him the chosen subject for an experiment meant to amplify both his power and his body.

Asked why he agreed, Nine at first declines to explain himself. Daruma counters that secrecy alone disqualifies him, so Nine talks. His Weather Manipulation lets him command the sky, and yet he describes himself as pinned to the earth. In his reading, nobody gets to choose anything from the moment they exist, and however much power is handed to a person, the society around them will grind them down regardless. He has watched others crushed the same way by the same superhuman order. The current world, he concludes, is simply a mistake. A genuine hero society would rank people by the supremacy of the Quirk they carry and nothing else, rendering the categories of Hero and Villain meaningless, and so he intends to return everything to the state it came from. Daruma, increasingly delighted, judges him a flawless candidate in both mind and body.

Later, Nine lies sealed inside a machine as the procedure begins. He has no real idea what it will do to him and feels certain anyway that it will work out, and he resolves that the ideal world he builds will belong to his teammates too, not only himself. As consciousness slips away, he never notices All For One and Tomura Shigaraki passing the room, nor Tomura's brief glance at him. His last thought is a promise to destroy whatever stands in his way.

Daruma Ujiko interviews Nine and draws out the reasoning behind his hatred of hero society, confirming him as the subject for an experiment to enhance his Quirk and body.

Nine's Weather Manipulation and the cellular deterioration it causes are established, along with glimpses of the oppression suffered by his future teammates.

Tomura Shigaraki passes Nine's operating room and glances at him, revealing that the two crossed paths before the main series, a detail earlier hinted at in Chapter 222.

A nine-page one-shot written and drawn by Kohei Horikoshi as a prelude to My Hero Academia: Heroes: Rising, included in the Volume Rising tie-in book. It appeared in Japan on December 20, 2019, with the VIZ release following on March 26, 2020.

Nine appears on the cover, and Chimera, Slice, and Mummy are introduced here.

Instead of a number the chapter is designated "Chapter ???". It is also the only movie tie-in chapter that was never adapted into an OVA for its film's Blu-ray release.