Ongaku No Shinigami

Ongaku No Shinigami summary:

Otoya Hibiki, a first year high schooler was chosen and invited to download a special game, called ’App battle’. Deleting the invitation, the game was forcefully downloaded into his smartphone and now, he has entered the world of ‘App battle’. But ‘App battle’ isn’t some normal game, but a game full of death and murder.