Transformation: Adamantine Staff

Monkey King Enma's transformation technique, which is characterised by the hardness of diamond and its at-will extendibility, the Adamantine Staff is the Hiruzen Sarutobi's weapon of choice, and with it in hand he's made it through a world of war for an extended period of time. This staff, when in use, was able to effortlessly destroy the wooden structures created by Hashirama, the original Wood Release user, and also to overwhelm Kurama, sending him to the borders of Konoha.

When Enma is transformed into the Adamantine Staff, it is possible for him to attack of his own free will using his claws and fangs which protrude from the staff itself, adding an element of surprise and irregularity. Forged in the fire of battle, its at-will transformation-based attack and defence go beyond the scope of proficiency, to attain the realm of shinobi "godhood". The main method of attack consists in strikes and thrusts that take full advantage of the ability to manipulate it's size and length.

The hardness of diamond also translates into destructive power. Despite its hardness, Enma stated that he could still suffer damage from the Kusanagi, wielded by Orochimaru.

Trivia

 * The concept of an extendible staff is based on the story of Sun Wukong and his Ruyi Jingu Bang.
 * A nyoi, which can be translated as priest's staff but literally means as one (a person) wishes, is an auspicious decorative object associated with Chinese Buddhist rituals. It was made from bone or jade, with a head shaped either as a cloud or as a lingzhi mushroom and had a slightly curved handle, but the production and use of these objects declined after the Tang Dynasty.
 * Vajra, which is in Japanese, is a mythical material that is said to be able to cut through any substance, but it itself, cannot be cut.