Sword Gai

A promotional video for the manga.

The story revolves around Gai, who was born beside a woman who met her death in a forest. He was adopted by the swordsmith Amon. A few years later, Gai becomes Amon's apprentice, and during the forging of a sword, he loses his right arm. To help him, Amon crushes the demon sword Shiryū (lit. Death Dragon) to form it into his replacement right arm. He then is able to fuse with the weapon on his arm, and fights against his enemies.

(Source: ANN)