A "master criminal" refers to someone who is extremely skilled and clever in committing crimes. They are experts in their illegal activities and are often successful in carrying out their plans without getting caught. Full definition
In Heat, they presided over parallel plot lines as master criminal and master detective and converged only late in the story. (shepherdexpress.com)
It's unknown how this character will be integrated into the game, but if it echoes the source material, he'll be a vigilante who begins eliminating Gotham's master criminals with a vengeance. (bytesizeimpressions.com)
Synopsis: Master criminal Neil McCauley (Robert De Niro) is trying to control the rogue actions of one of his men, while also planning one last big heist before retiring. (mrqe.com)