SWORDGAI THE ANIMATION

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 Shiryuu (lit. Death Dragon) to form it into Gai's replacement right arm. Gai then is able to fuse with the weapon on his arm, and fights against his enemies. (Source: Anime News Network) — Description from AniList community (anilist.co).