Decay allows the user to destroy anything their hands touch, making it crumble to dust. Much like is the case with Eri's Rewind, it is the result of a rare mutation that no one was aware of before it manifested.
Whether the target is organic or not, Decay will work without fail. As such, this Quirk is extremely lethal against living beings, having their flesh rot away if hit by it. The speed of the disintegration has developed from slow to almost immediate, eventually spreading through their whole body if a victim doesn't amputate the decaying body part (Eraser Head was able to avoid this by erasing Decay before it could cause further damage to his flesh).
It is also possible for the effect of Decay to spread beyond what the user has touched, creating a domino chain of disintegrated victims. Tomura has had this ability ever since Decay first manifested on him, being strong enough to destroy an entire household, but due to his jumbled up memories of the event, he seemingly forgot about or mentally repressed this specific ability until halfway through the Revival Celebration, where flashes of his past started reminding him of what happened. At this early point of Tomura's life, Decay's disintegration process was far weaker as it reduced affected targets down to chunks instead of dust.
Because the Quirk requires Tomura to touch a solid object in order to be activated, it is possible to counter Decay through Quirks that allow for the manipulation of fluids or particles, which Tomura cannot properly touch with his fingers.
In addition, this Quirk is not effective if the user is wearing a pair of gloves.
Due to the fact that the user's Quirk Factor lies within their hands, more specifically their fingers, Tomura would lose the use of Decay if he lost them, similar to Overhaul.
Decay first manifested in five-year-old Tenko as he played with his dog, Mon. Tenko felt a shock through his left arm moments before getting physically punished by his father. While embracing Mon, Decay's effects materialized for the first time, resulting in Tenko accidentally killing his dog and then, later, his sister. Eventually, Tenko, who was resentful about his family and in a manic state, destroyed the rest of them as well as their household with his newfound Quirk.
All For One took advantage of this event in order to recruit Tenko (now Tomura) as his pupil and eventual successor when the latter was left alone and confused after the death of his family. With the help of his doctor and assistant, All For One was able to salvage the hands of the family members Tomura killed, using them as a sort of tool to help Tomura deal with his loss while simultaneously increasing his feelings of rage and hatred, which All For One believed was necessary for his plan to raise Tomura and his Quirk's strength.
As Tomura's memories of killing his own family were repressed by his subconscious, he became unaware of some aspects of his Quirk, unconsciously holding himself back from causing more destruction than he was actually capable of. As his past started sorting itself through his mind due to damage caused to his family's hands, Tomura started relearning and showcasing how truly powerful Decay is while battling against the Meta Liberation Army.
Decay is Tomura's main method of combat. By nature, this Quirk forces Tomura to always engage his opponent in close quarters, though he often requires only to make contact with his enemy in order to kill or heavily injure them. Tomura seems to have improved his Quirk ever since its first appearance at the Unforeseen Simulation Joint, as Decay's disintegrating speed has become noticeably faster in subsequent usages.
Tomura is often careful with his hands when touching objects he doesn't want to destroy, letting at least one of his fingers free from use while holding things. He employs a similar method when holding people hostage, threatening to disintegrate them if they don't comply or cooperate with him. If Tomura gets angered or frustrated, he is likely to drop his carefulness.
Through vigorous training via several weeks of combat against Gigantomachia, Tomura has greatly improved the strength of his Quirk. Decay works almost instantaneously now rather than taking several seconds before a target crumbles to dust and continues to evolve under high-stress environments. Using his hands everything he touches is affected by Decay's spread; the function of which working in a solipsistic fashion where anything or anyone connected to what he's touching suffers its effects.
Even with his left hand partially destroyed, Tomura was able to decay part of Re-Destro's right index finger using only his left hands two remaining digits, albeit the process was much slower and weaker compared to when he uses all five fingers. By the end of his fight with Re-Destro, Tomura grew yet again, and his Decay is now strong enough to reduce nearly half a city to an ash cloud.
After All For One's Quirk was surgically grafted into Tomura's body, Decay became even stronger than before: its speed and range of effect increased drastically and was now not only able to spread the decay on touch, but just by simply placing his hand on the floor, Tomura was able to destroy Jaku General Hospital while decaying everything inside, before spreading to the nearby city. It has grown to the point anything touched and disintegrated spreads the effect even after parts of the crumbling matter break away and touch someone.
Tomura also has control over what specifically gets affected by Decay, as he was seen wearing X-Less's cape despite the hero being disintegrated. This enhanced control also allowed him to avoid disintegrating many of the Nomu in Dr. Garaki's laboratory, though he admits he couldn't save all of them.
- Decay is very similar to Overhaul in practice, but there are some key differences:
- While both abilities are capable of destruction, only Overhaul has access to reconstruction, giving it more natural versatility.
- Decay's effects can spread to anything connected to a target. Overhaul's effects are limited to the target only.
- Against a human opponent, Decay's effects will keep quickly spreading throughout their body with no way of stopping it save for mutilation of the affected area. Overhaul's effects happen instantly and can be contained within a specific body part.
- My Hero Academia Manga and Anime: Chapter 222 and Episode 108.
- My Hero Academia Manga and Anime: Chapter 227 and Episode 109.
- My Hero Academia Manga and Anime: Chapter 69 and Episode 38.
- My Hero Academia Manga and Anime: Chapter 233 and Episode 111.
- My Hero Academia Manga and Anime: Chapter 234 and Episode 111.
- My Hero Academia Manga and Anime: Chapter 239 and Episode 112.
- My Hero Academia Manga: Chapter 275.
- My Hero Academia Manga: Chapter 272.
- My Hero Academia Manga: Chapter 276.
- Decomposition - Wikipedia article about the concept behind Tomura's Quirk.