Yagate, Hitotsu no Oto ni Nare

He fell in love with the piano, and it became a curse... Seven years after Kanade, the once acclaimed "boy genius", gave up playing the piano due to an incurable disease, his old rival who's now a famous pianist appears before him. Reunited with the person he has longed to see yet never wanted to meet, Kanade is forced to face the music once more - with only his left hand as his weapon.

(Source: Shogakukan, translated)