Shikotsumyaku

is a kekkei genkai  which gives its wielder the ability to manipulate their own skeletal structure (their osteoblasts and osteoclasts). By infusing their calcium with chakra, they could manipulate the growth and properties of their bones to their liking. Kaguya Ōtsutsuki manifested a similar, albeit more lethal, ability to weaponise her bone structure from which Shikotsumyaku originates. This kekkei genkai is inherited into a rare few amongst Kaguya's descendants in the Kaguya clan, Kimimaro being the only other known inheritor of this power. Apparently, this ability gives the user a unique skeletal structure, as when Kimimaro was fatally ill, Kabuto Yakushi claimed there had been too little information on his body to give any medical treatment.

After integrating Kimimaro's DNA into himself, Kabuto gained access to this kekkei genkai.

Abilities
The main capabilities of Shikotsumyaku is allowing the user to manipulate the speed of their bones' growth as well as the location of calcium deposits allowing for great versatility. This allows them to create weapons of bones that can either protrude from any part of the body, or pulled out and used like handheld weapons. The user was also able to create armour of dense bone underneath their skin. They can even fire pieces of their bones as long-ranged projectiles.

Any bone that they remove from the body immediately regenerates, as does the skin that is damaged when the bones are removed. Although it seems it takes a bit longer for the limb to become usable again, unless the user regrows the bones consciously. Users can increase the density of the created bones, making them stronger than steel. This not only makes the created weapons very powerful, but also makes their bodies virtually indestructible; the bones can withstand even a chakra-infused blade, which is usually enough to cut through anything. The bones structure can also be hollowed out completely as shown by the copy of Kimimaro which Kabuto had create a makeshift flute during his battle with the Uchiha brothers.

From what two of the known users of this ability, Kimimaro and Kabuto, have shown, most of the techniques are named after flowers. Most of them are also taijutsu-based sword dances, which are used for close combat and defence. However, as Kimimaro demonstrated, the bones can also grow quite a large volume, and likely grow from each other, as a form of a ninjutsu, and that he can emerge from any of these bones.

Orochimaru coveted this ability greatly, stating the kekkei genkai granted the user an impenetrable skeletal frame, which can resist any type of physical attack. In addition, should a Shikotsumyaku user ever go on the offensive, they could instantly turn their bones into the sharpest spear. Because of this, the Shikotsumyaku is held in high regard as the ultimate taijutsu ability.

Trivia

 * 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.