Blue Exorcist

There are humans and then there are demons; two sides of the same coin, as are assiah and Gehenna (two different worlds).  The only way to travel between the realms is by the means of possession, kind of like what you hear in ghost stories.  However with Satan, ruler of Gehenna cannot find a suitable host to possess, and therefore remains imprisoned in his own world. So in a desperate attempt to conquer Assiah he sends his son instead, intending for him to grow into a vessel capable of possession by the demon king.


So anyway Ao no Exorcist or Blue Exorcist follows Rin Okumura (main character) and his twin Yukio who never knew who their real father was, and they were also raised by a man named Father Fujimoto, a famous exorcist.  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!  After a terrible accident Rin swears to defeat Satan, but to do that would mean entering the mysterious True Cross Academy and become an exorcist himself.