“Akuma” - living weapons with soul inside them. They are made by the Earl, who lived a thousand years. He tricks people who lost their loved ones into bringing them back to life. To do that, he needs them to call out the spirit of their loved one and put it into the vessel. But it only becomes something not human, something that the Earl uses to kill people and turn them into another akuma.
The exorcists are chosen holy people who are the only one to stop these weapons. They are equipped with “innocence”, the only weapon that can destroy the akuma. The souls inside the weapon are released by the innocence and the akuma will be destroyed.
Allen Walker, the protagonist, is boy who turned his father into an akuma. He later joined the Black Order to become an exorcist to repent for his sin. He then encounters different people, akuma, and exorcists.
This is a really great series by Hoshino-sensei. It beautifuly portrays the psychology of human beings with how they react in times of despair and joy, and tries to explain that “nothing is perfect”. Though you have to watch/read the whole series to be able to grasp the essence fully.