Regeneration

Regeneration is a supernatural ability possessed by demons that allow them to rapidly heal injuries.

Overview
Regeneration is the ability to heal from one's own injuries at an extremely fast rate, an ability which is shared and possessed by all demons. Their regenerative prowess enables to heal from all kinds of damage, from small cuts, to severed limbs, to pulverization and even decapitation. A demon's regeneration usually activates immediately after any sort of damage is inflicted and can take anywhere from a handful of seconds to just mere moments to completely recover. There doesn't seem to be a limit to what or how much a demon can regenerate, thus making them essentially invincible.

All demons' regeneration seem to linked to their physiology, that is the amount of Muzan Kibutsuji's demon-transforming blood they have assimilated, with more blood assimilated resulting in faster and more potent regeneration. Hence, stronger demons, like members of the Twelve Kizuki, have overall more powerful regenerative abilities than weaker demons.

Generally, regeneration functions as a sort of immortality for demons, as no amount of damage inflicted to them could permanently incapacitate or kill them. Even lesser demons such as the Temple Demon, didn't die after his head was kicked off by Nezuko Kamado, and his regeneration was potent enough for his severed head to begin regrowing his limbs. Stronger demons, such as Yushiro, can recover from having his brain completely smashed to bits in a matter of seconds.

As demons become more powerful, their regeneration speed increases proportionally. Non-Twelve Kizuki demons such as the Father Spider Demon took several seconds to regrow his left arm, while Akaza, an Upper Rank, regrew both of his arms faster than the trained eye can see, making it seem like it happened instantaneously. After being decapitated, Hantengu's body regrew a head instantly while his severed head regenerated an entire new body in around the same time. Muzan himself can regenerate as he is getting injured, tricking even the enhanced perception of Marked Hashira into making them think they never injured him in the first place.

Though it may be a trait unique to her, Nezuko Kamado has displayed the ability to reattach her body by controlling the blood that connects them.

While mostly speculative, its been implied that demons can control their regeneration up to a certain extent, either forcefully slowing down their recovery speed or stopping it completely.

Weaknesses
A demon's regeneration is not perfect. The one key limitation to a demon's regeneration is that they are unable to recover from damaged caused by sunlight, thus permanently killing demons. However, if their whole body isn't incinerated by sunlight, it is implied that they can still heal.

The Demon Slayer Corps has weaponized this weakness through the use of Nichirin swords which, through their extensive absorption of sunlight, allows Demon Slayers to cause wounds that are more difficult to heal from. However, Nichirin swords can only permanently kill demons if they completely cut through and sever the neck of demons. It is suggested that this method works because their regeneration is the weakest at their necks, and therefore the introduction of sunlight, in the form of Nichirin swords, in that vulnerable area would be able to permanently kill demons.

Nichirin swords are made even more deadly in the form of Bright Red Nichirin Swords, which have extreme regeneration-hampering properties, even on parts other than the neck of demons. One of the most prominent showcases of this trait is when Muzan took several seconds to heal his severed arms while it would normally be healed the moment it was severed. Wisteria poison is also a tool that can be used to permanently kill demons when inflicted on various points of their body. While stronger demons may resist and adapt to its effects, a large enough dosage can slow down and negate the regeneration of the likes of Doma, Upper Rank Two.

Muzan Kibutsuji himself has also shown the ability to forcefully stop the regeneration of demons and permanently kill them either through cellular destruction or the Curse of Kibutsuji.

Levels of Regeneration
Enhanced Regeneration: The standard level of regeneration possessed by lesser demons to the Lower Ranks of the Twelve Kizuki. Their regeneration is fairly potent, taking only a few seconds to activate and just as much time for the damage to completely heal, albeit more severe injuries would take longer.

Immense Regeneration: A more advanced level of regeneration, which is possessed by the demons among the Twelve Kizuki's Upper Ranks. Their incredible regenerative abilities activate the moment they are injured and usually finish healing in the same moment as well depending on the severity of the damage inflicted. They are able to survive wounds even the regeneration of lesser demons would not be able to combat, such as wisteria-based poisons and attacks from the Hashira. While mostly speculative, it is shown that the strongest Upper Ranks can overcome decapitation by Nichirin swords and regrow their heads, presumably if they have a strong enough will to live.

Nigh-Absolute Regeneration: The highest level of regeneration shown in the series, which is only possessed by the Demon King himself, Muzan Kibutsuji. This level of regeneration enables him to heal while he is being damaged, and also makes him immune to the decapitation using Nichirin swords. Muzan's extreme regeneration posed such a threat, a special drug had to be developed in order to weaken it by rapidly aging him and forcing his body to expend energy to fight off its effects. After assimilating Muzan's blood, Tanjiro Kamado, has also shown to possess an even higher caliber of regeneration, becoming him immune to both sunlight and Bright Red Nichirin Swords.