Orochimaru's Juinjutsu

Orochimaru's juinjutsu is a transformation cursed seal that Orochimaru can apply to others in order to make them stronger.

Overview
Orochimaru's body wasn't strong enough to handle Sage Mode, prompting him to start looking for an alternative. He was at some point approached by Jūgo, who wanted to find a cure for his bodily transformations. By studying Jūgo's body, Orochimaru discovered that Jūgo's body produces an enzyme that causes him to take in natural energy, creating senjutsu chakra. By isolating this enzyme and mixing it with his own chakra, Orochimaru created a cursed seal he could brand others with. Orochimaru typically gives his cursed seals to either his more powerful followers or those he would like to use as his host body.

To apply a cursed seal, Orochimaru bites the recipient's neck, causing the seal to appear near the puncture wound. Recipients are rendered unconscious while their body either accepts or rejects the cursed seal. From extensive practice, Orochimaru established a 10% compatibility rate for the cursed seal; those whose bodies aren't compatible with the cursed seal will die. If they do survive, the recipient will have access to the cursed seal's power boosts, access that grows through repeated use. If unused, cursed seals will grow dormant, though Orochimaru can forcefully activate them if he gains access to them.

Cursed seals seen in the anime tend to have fatal consequences even if the recipients survive the initial application. Mizuki's seal, which was a failed prototype that required extra procedures to activate, nearly kills him due to its incompleteness and causes him to lose all his powers. The cursed seals administrated to Team Guren kill them if they do not take the required medication periodically, preventing them from escaping Orochimaru's clutches. On the other hand, the seals given to the Iburi clan partially stabilised their otherwise unstable ability.

Once applied, removal of the cursed seal is apparently difficult. Kakashi Hatake and the Third Hokage have on separate occasions opted to suppress the cursed seals rather than remove them outright. The only times the cursed seal have been fully removed have been as a side-effect of removing Orochimaru's consciousness from the recipient's body, fragments of which are embedded in each of the cursed seals: Itachi Uchiha does so with the Sword of Totsuka; Sasuke Uchiha does so with the Evil Releasing Method. So long as part of his consciousness remains in a cursed seal somewhere, Orochimaru can never truly die.

Mechanics
When they first receive the cursed seal, recipients cannot control their access to it, and as such they draw on its power unwillingly. Until they become accustomed to it, use of the cursed seal will cause them intense physical pain, though they may be able to briefly ignore this. While active, the cursed seal feeds off their chakra and replaces it with Orochimaru's own senjutsu chakra, increasing their chakra levels and making their jutsu stronger. So long as they have at least some chakra of their own, this gives them a back up reserve that they can call on, though if not employed carefully this may have fatal consequences. Prolonged use of the cursed seal corrodes the mind, making them more susceptible to Orochimaru's influence. In exchange, the cursed seal synchronises with their body, exponentially enhancing the abilities of their normal state.

Recipients initially only have access to the cursed seal's. When active, the seal's design spreads across their body from its point of application, glowing and replicating as its coverage grows. The degree to which this occurs is up to the user, and the degree to which it spreads is proportionate to the overall effect it has.

In order to increase the the advantages of using the cursed seal, as well as to eliminate some of the risks and discomfort that come from using its first level, Orochimaru developed Mind Awakening Pills to advance cursed seals to their next stage. Under normal circumstances, the pills kill those who take them, requiring that they be placed in a near-death state after taking the pill while their body acclimates for the heightened transformation.

The of the cursed seal causes the user's body to physically transform. Called a, it is similar in appearance to Jūgo's transformations: they gain yellow irises and black sclerae, altered skin colour, and longer hair. When in this state, the seal eats away at the mind and personality of its user, driving them to madness if used for too long. It also takes quite a toll on their body since they are being mutated by the seal. Skilled users of the cursed seal are able to isolate transformations to specific parts of the body.

Seals
Orochimaru has various cursed seal designs, some being stronger than others. Second level transformations are always the same for a single user, but it's not known if the same cursed seal causes the same transformations in different users.
 * Cursed Seal of Heaven: Worn by Sasuke Uchiha and Anko Mitarashi.
 * Cursed Seal of Earth: Worn by Kimimaro.
 * Sound Four's Cursed Seals: Worn by Jirōbō, Kidōmaru, Tayuya, Sakon and Ukon.
 * Prisoners' Cursed Seals: Worn by Ahiko, Guren, Takishi and Tsukushi.
 * Animal Cursed Seal (anime only): Worn by Mizuki.
 * Team Guren's Cursed Seals (anime only): Worn by Nurari, Kihō and Kigiri.
 * Iburi Clan's Cursed Seals (anime only): Worn by Gotta and Yukimi.

Trivia

 * Compatibility with one of Orochimaru's cursed seals in turn means one is compatible with Jūgo, allowing him to transplant his own flesh and chakra into their bodies.
 * Orochimaru can apply other cursed seals by biting an opponent's neck. These other cursed seals temporarily bind the target upon application, spreading across their body to restrict their movements. This is method of application and the possibility of rejection by the host are similar in nature to how the White Snake Sage teaches senjutsu.