Ao no Exorcist: Shimane Illuminati-hen

At True Cross Academy, Rin Okumura unknowingly comes across the first signs that something is amiss; ordinary people have been gaining the ability to perceive the invisible creatures that roam the human realm, Assiah. While Rin sees this as an opportunity to make a new friend, his twin brother, Yukio, is more burdened than ever. Swamped with his education, exorcist duties, and teaching position, Yukio has remained silent about his awakened powers, unable to understand their scale or significance.

While Rin and his friends prepare for the annual Exorcist Certification Exam, the True Cross Order has uncovered the first artificial gate to the demonic realm of Gehenna. Witnessing its size and the advanced technology that made it possible, the exorcists realize they are now dealing with the Illuminati, an extremely wealthy and powerful organization. Moreover, the inscrutable Illuminati have planted three spies in the True Cross Order—and one may be hiding among the aspiring exorcists sworn to protect mankind from the demonic threat.

(Source: MAL Rewrite)