Regeneration

Present from Chapter 2 and the second anime episode, regeneration is the passive ability shared by all demons to mend their own wounds extremely quickly. It can repair anything from small cuts to lost limbs, pulverized flesh, and even severed heads, usually activating the instant damage is dealt and finishing within seconds. There seems to be no ceiling on what a demon can restore, which renders most of them effectively unkillable. Because demon clothing is often grown from their own flesh, damaged garments regrow as well.

A demon's healing is tied to physiology, namely how much of Muzan Kibutsuji's transforming blood it has absorbed; more assimilation yields faster and stronger recovery. As a result, members of the Twelve Kizuki outheal weaker demons by a wide margin. Even lowly demons show the trait, as the Temple Demon survived having his head kicked off and began regrowing limbs, while the stronger Yushiro recovered from a completely shattered brain in seconds. The ability can also override a former human's terminal illness or fatal injuries, and in Nezuko Kamado's case extends to reattaching severed parts by controlling the blood between them.

Levels and control

Healing comes in tiers. Enhanced regeneration is the standard grade for lesser demons through the Lower Ranks, taking a few seconds to work. Immense regeneration belongs to the Upper Ranks, kicking in the moment they are hurt and often finishing instantly; at the top end it can even overcome decapitation by a Nichirin sword and regrow a head given enough will to live. Nigh-absolute regeneration is unique to Muzan, who heals while being struck and is immune to Nichirin decapitation. Demons can also throttle their own healing, slowing or halting it, and discarded parts that are no longer needed dissolve on their own.

Weaknesses

The power has limits. Damage from sunlight cannot be healed and permanently kills demons, though a body not fully incinerated may still recover. The Corps weaponizes this through Nichirin swords, which store sunlight and inflict wounds that are harder to mend; such a blade kills permanently only by fully severing the neck, the spot where regeneration is weakest. Bright Red Nichirin Swords go further, hampering healing even away from the neck. Wisteria poison can slow or negate recovery in large doses, and a special drug developed by Tamayo, Shinobu Kocho, and Yushiro weakened Muzan by rapidly aging him. Hantengu's manifestations also have their own quirks, with delayed healing when their tongues are cut.

Every demon regenerates, but the degree varies sharply by strength. Lesser demons like the Temple Demon and the Spider Demons heal slowly by comparison, Yushiro recovers from devastating head damage, and Nezuko Kamado uniquely reattaches her body. Among the Upper Ranks, Akaza regrows both arms faster than the eye can follow, Hantengu regenerates entire bodies, and Doma and Gyokko show rapid recovery. Muzan Kibutsuji wields the absolute peak, healing as he is wounded, and Tanjiro Kamado later gains an even higher caliber after assimilating Muzan's blood.