Episode 12

"The Most Terrible Demon King" is the twelvth episode in the anime of The Reincarnation of the Strongest Exorcist in Another World.

Plot
Principal elects Seika as Student Representative to welcome the next first year students. Seika decides to abandon his fear of drawing attention, after dealing with the approaching demons. The demons arrive at the academy to slaughter the hero, but find the academy covered in a confusing red fog. Zor decides to abandon the mission, only for Seika to appear. Zor, who can see peoples power, is terrified by Seika who beheads Muderev easily. Pyrslaria’s evil eye is defeated by demon servant White Snake, whose own evil eye stops her heart.

Lo-Ni attempts to tame White Snake but is eaten. Zor reveals Seika bears the title Most Terrible Demon King, hoping Gull-Ganis will escape to tell the other demons the prophesied Demon King is an ally of humanity. Zor is obliterated by Seika’s fire magic and Gull-Ganis teleports away but dies when Seika uses a substitution talisman to stab him through the heart. Mabel, who saw everything, promises Seika she will keep it secret. At the ceremony to welcome new students the Dirac Knights of Marquis Grevil announce Amu is under arrest for murdering a demon emissary who, at the time of his death, was visiting the academy under diplomatic immunity.

Trivia

 * Despite Zor seeing the title Seika bares, it's unknown if this is just for his control of ayakashi (which are demons, technically) and the genuine Demon King is still out there. Seika himself would never care to be the Demon King; it would go against his entire plan to live a peaceful life.