Sword Gai: The Animation

Gai is a young man who worked as an apprentice for a blacksmith named Amon after the latter discovered him as a newborn clutching the fabled cursed katana known as "Shiry" that compelled his mother to commit suicide rather than give in to its raging appetite. The Chrystalis progressively lose their humanity while evolving into armored monsters known as Busoma, and the katana is revealed to be one of several cursed weapons that convert the user into a thrall to embody their homicidal instincts. Gai lost his right arm after an attempt to extinguish Shiry went wrong, so Amon transformed the sword into a prosthetic limb for him. Gai fights the Busoma, who want to eradicate humanity, while transforming into a chrysalis with the capacity to fuse with his weapon.