While My Hero Academia season 5 details how antagonist Tomura Shigaraki (Eric Vale) first manifested his quirk Decay, one theory suggests that the quirk actually belonged to Deku first.
During Shigaraki’s fight with Re-Destro in Deika City in episode 23 «Tenko Shimura: Origin,» My Hero Academia finally reveals through a series of disturbing flashbacks the events of Shigaraki’s childhood that catalyzed his journey to becoming All For One’s successor, but the coincidental nature of his tragic
. Read more on screenrant.com