Shikotsumyaku

is a kekkei genkai which gives its wielder the ability to manipulate their own bones. It very rarely manifests within members of the Kaguya clan, of which Kimimaro was the last survivor.

Overview
Kaguya Ōtsutsuki possessed the ability to manipulate her own skeletal structure, which she most commonly employed via her All-Killing Ash Bones. The Kaguya clan's Shikotsumyaku is a similar, but less lethal, power. Users of Shikotsumyaku possess three unique skills: the ability to control their osteoblasts (cells responsible for producing bones); the ability to control their osteoclasts (cells responsible for breaking down bones); the ability to regulate their bones' calcium density. So long as their body does not tire out, users can take advantage of these abilities without limit.

With Shikotsumyaku, users can grow new bones anywhere in their body. Typically these bones are functionally and visually identical to other bones in the body, such as the spine or ribs. Other times the bones will be similar to standard bones, but with specific alterations: Dance of the Camellia uses an upper arm bone that has been sharpened for use as a blade. If the user chooses, they can create entirely original bones with no clear parallels: Dance of the Clematis: Flower uses a large, drill-like bone growth. The user's bones can be made denser, increasing their cutting power and rendering them resistant to crushing or cutting. If the user chooses, hollow bones can be produced, such as for use as a flute.

Users can manipulate their bones after or even as they're being formed, causing them to break through the skin. Most often these bones only protrude from the body, enough for the user to grab and remove them or, alternatively, remain jutting out for whatever offensive or defensive purpose the user intends. If the user chooses, the bones can be forcefully ejected from the body, as in the case of Ten-Finger Drilling Bullets. After the bones have either left the body or been retracted into it, the broken skin heals rapidly. If, in the case of removed bones, the user does not already have a duplicate prepared, the relevant body part will be misshapen and unusable until a replacement is produced.

Shikotsumyaku gives its users a huge advantage in combat, as they can produce an endless number of bones to suit any conceivable circumstance. Rarely do users encounter real opposition in a fight, for which reason their path through a battlefield is often left littered with dead bodies. On those infrequent occasions when they cannot quickly finish off an opponent, their ability to strengthen their own bones renders them highly resistant to damage, their bodies being likened to a fortress. Due to Shikotsumyaku's unprecedented power and versatility, the Kaguya clan believed themselves to be invincible. This overestimation of themselves led the Kaguya to pick a fight with a superior power, Kirigakure, causing them to be wiped out.

Orochimaru, in his quest for immortality, wanted to acquire Shikotsumyaku for himself by taking over Kimimaro's body, but Kimimaro became ill before Orochimaru had the chance. Kabuto Yakushi attempted to heal Kimimaro, but his kekkei genkai made his body so radically different from an average person's that, without sufficient information about his unique physiology, there was nothing Kabuto could do for him. After Kimimaro's death, Kabuto is able to replicate Kimimaro's Shikotsumyaku within his own body. Orochimaru subsequently gains Shikotsumyaku by absorbing the ability from Kabuto.

Trivia

 * Kimimaro refers to many of his Shikotsumyaku-dependent jutsu as "dances". Each dance is named after a different plant.
 * In the manga, when Kimimaro extracts bones from his body, his skin rips wide open. In the anime, to censor the gore, small holes are opened when bones come out, which are big enough to allow the bones to fluidly pass through the skin.