Orient 4

The only weapon that can vanquish a demon, and only those who overcome its "blade test" can become samurai and face off against the demon gods. However, something strange happens to Musashi when he takes the blade test. A mysterious shadow dwelling within him transforms his soul into black crystals, dredges up his past, and forces him to confront the childhood memories that he had buried deep within his heart...

Volume 4 of the Orient manga series by Shinobu Ohtaka.