Shikotsumyaku

is the kekkei genkai of the extinct Kaguya clan, which gave them 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. The ability seems to be rare amongst the clan members, as Kimimaro was the only one to exhibit it out of his entire clan.

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.

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. This allows them to create weapons of bones that can either protrude from any part of the body, or pulled out and used like hand held weapons. Immediately after the bone is pulled out, the broken skin heals. Although it seems it takes a bit longer for the limb to become usable again or unless the user regrows the bones consciously. 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. 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.

From what the only known user of this ability, Kimimaro, has 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. Overall, the Shikotsumyaku is considered to be the ultimate taijutsu ability, which is held in high regard.

Trivia

 * In the manga, when Kimimaro extracts bones from his body, his skin rips open wide. In the anime, to censor the gore, small holes are opened when bones come out, which are big enough to allow the bones to pass through the skin, but no more.