In the late Edo Period, a demon known as the Impure King killed thousands of people. After defeating the demon, the Knights of the True Cross kept its left eye safely sealed away on Academy grounds - but now someone has stolen it. Hearing the thief has taken a child hostage, Yukio and Rin go to help. The investigation and pursuit will lead Rin and his friends to Kyoto and involve them even deeper in a sinister plot. But will his friends' knowledge that Rin is the son of Satan drive a wedge between them?
This is based off of, I believe, episode 18 and what a better ending could have been. I love both seasons/series and would highly recommend you watch both. This season is less popular but that could be because it has been on very little streaming devices. Now it's on Netflix as the second season to Blue Exorcist. Hopefully it'll be less underrated.