Prolouge
The Thousand Year War was over, the Earl had been defeated, the Noah had stopped fighting under claims of having been manipulated by the Earl. Most of the Exorcists had died in the battle, the few who were still alive stayed with the Order so they could destroy any remaining akuma. There were a few exorcists that disappeared entirely after the battle, namely, Exorcist General Kanda Yuu, Exorcist Lavi Jr Bookman, and Exorcist Allen Walker. The Order searched for them for three years, Lavi, Jr Bookman was found to have taken over as the new Bookman, as the previous Bookman died shortly after the war. Kanda Yuu was never found, though reports suggest that he is still alive.
Exorcist Walker was proclaimed dead, however, no body was found. The Vatican declared that if he were to be found he would be tried for heresy on account of being a Noah, and other supposed crimes committed during the war. There was quite the uproar and while they couldn't openly oppose the Vatican, many of the exorcists, scientists, and finders decided that if Walker was ever spotted nothing was to be done. In their eyes Walker had earned his retirement, they weren't about to bring him in so he could be executed.
Years passed, the Black Order disbanded, and the akuma were gone, the plant and its remains were disposed of, and the Noah disappeared. More time passed, and something shifted. The akuma were back, but they weren't the same. These were different, they were strange creatures. Most were small, these new demons, Hobgoblins, Coal Tar, Shades, Ghouls, and Ghosts were the first to appear. Then they found out about Satan and the Eight Demon Kings. Of course, none of this was actually new, the Knights of the True Cross had existed for many years, almost as long, if not longer than, the Black Order. Many had brushed it off as superstition in favor of the more urgent threat, but with the akuma gone, people flocked to the Knights of the True Cross like never before.