Chakra Blades

A is a special-made metal that can be infused through "chakra flow" with elemental chakra or even Yin–Yang chakra to produce added effects. According to Shikamaru, the blades are able to absorb the chakra type of the user, and become the origin of a technique, for the user.

This type of weaponry was a specialty to Asuma Sarutobi. His particular blades are a uniquely-designed trench knife worn like brass knuckles, wearing the blades above his knuckles with a zigzag-shape for each knuckle, giving each blade "teeth". By coupling these knives with his taijutsu, Asuma could strike with devastating effect, such as taking out nine Otogakure ninja in a matter of seconds. When infused with wind-natured chakra, they can easily pierce and cut through stone and even metal. The knives' sharpness and effectiveness can also be maximised if the person has better control of their chakra as seen with how Asuma was training Sora and Naruto Uzumaki in focusing their wind chakra into their kunai. After Asuma's death, the blades come into Shikamaru Nara's possession, which he used by infusing his chakra and striking their shadow with it to completely immobilise them.

Trivia

 * Naruto is seen using chakra blades in the non-canon Naruto x UT OVA.