This RP would be set along the same time as the manga and anime, though it wouldn't have to have the same plot. Mainly the lives of some exorcists.
Unknown to mankind there are demons known as Akuma in this world. They are killing machines made by The Millennium Earl. To combat Akuma there are exorcist, people chosen by innocence. Innocence is a substance capable of killing Akuma. An exorcists job is to kill Akuma and find more innocence.