Decay allows Tomura to destroy anything his hands touch, making it crumble to dust.
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 varied from slow to almost immediate, eventually spreading through their whole body if a victim doesn't amputate the decaying body part (Shota Aizawa was able to avoid this by erasing Decay before it could cause further damage to his flesh). Tomura primarily activates Decay through his fingers, but his feet have been shown to be capable of triggering his Quirk too.
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 Meta Liberation Army Arc, where flashes of his past started reminding him of what happened. At this point of Tomura's life, Decay's disintegration process was far weaker as it reduced affected targets down to chunks instead of dust, meaning it was still incredibly deadly to living beings anyway.
Decay will only occur when a Tomura makes contact with the target, indicating that its activation isn't voluntary, or at least that Tomura's control is clumsy. Because the Quirk requires Tomura to touch a solid object in order to be activated, it is possible to counter Decay through Quirks that allows 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.
Decay first manifested on Tenko at age five 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, intentionally destroyed them as well as their household, but wouldn't recall this memory until the Meta Liberation Army Arc.
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. 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 increase his feelings of rage and hatred, which All For One believed was necessary for Tomura and his Quirk to grow stronger and further his plans.
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. Even with his left hand partially destroyed, Tomura was able to decay part of Re-Destro's hand using only two fingers, albeit the process was much slower and weaker compared to when he uses all five fingers.
- ↑ My Hero Academia: Battle For All Official Website
- ↑ My Hero Academia Manga: Chapter 222.
- ↑ My Hero Academia Manga: Chapter 227.
- ↑ My Hero Academia Manga: Chapter 222.
- ↑ My Hero Academia Manga and Anime: Chapter 69 and Episode 38.
- ↑ My Hero Academia Manga: Chapter 234.
- Decomposition - Wikipedia article about the concept behind Tomura's Quirk.