Blue Exorcist, Vol. 3 by Kazue Kato

Rin and his exorcist classmates are caught in a secret war against the forces of darkness.

Raised by Father Fujimoto, a famous exorcist, Rin Okumura never knew his real father. One day a fateful argument with Father Fujimoto forces Rin to face a terrible truth—the blood of the demon lord Satan runs in Rin’s veins! Rin swears to defeat Satan, but doing that means entering the mysterious True Cross Academy and becoming an exorcist himself.

As a new Exorcist Exwire at True Cross Academy, Rin Okumura has a long way to go before he reaches the top rank of Paladin. When Rin and his classmates are sent to search for a ghost, Amaimon, King of Earth, takes Rin by surprise and steals the Koma Sword. Higher powers have been observing events at the academy and make their presence known. New players enter the game, and Rin doesn’t even know he’s playing!