Living in a big city where everyone is always in a rush to get around, a place full of people you may only live once, you may not have realized that some of these people may be associates of the Devil.
They lurk around the city, looking for people who require assitance. Maybe help through emotional turmoil, or maybe helping them achieve what they desire most. Once these people have been found, they write up a contract. The people get what they want, but in exchange they must give up their soul when the term stated in the contract is over. But rather than being damned to the depths of hell, there is a possibility that you may become something else. In general, people would run, screaming 'monster', but term is a bit broad. These monsters lose their minds and rampage across town, causing chaos. The form they take vary among people, but the results are generally the same. These days they take human form if they're lucky, but they always have certain abilities. There's a wide range of abilites, so I won't bother listing them down. We call these monsters 'Schiff'.
And where there is chaos, there will always be someone to clean it up. When you hear the word wizard you would imagine an old man with a beard, a witch would be an evil woman on a broomstick, I wouldn't even know how people would describe warlocks and magi. But all these magic users live among us, trying to stop the Devil's associates, and clean up the chaos which they leave behind. Many magic users have been scouted by a government funded orgainization now known as the 'Ministry of Magic'.