Walter.

In Stephen King's book, The Gunslinger, the man in black is the antagonist of the story where Roland, the world's last gunsinger, is persuing this man across a cyclopean desert, only to kill his own "symbolic son" Jake in a desperate attempt to catch the man in black, and learn more about his own dark future. . .

We learn more of Walter in the fourth book in the Dark Tower series, Wizard and Glass. He is sent before a contingent of John Farson's troops, led by George Latigo, to the town of Hambry, where Farson had a glass ball sent for safekeeping. The ball is part of Maerlyn's Rainbow, and allows he who holds it to see things. . .but never anything good.

Walter is described as something close to a wizard, but more like a "glamor-man"; an illusionist, a sorcerer, and at times, a fortune teller. His appearance can shift back and forth between his own countenance and that of people who have been known by those to whom he is speaking. He wears a long black robe, similar to a priest's cassock, has closely cropped hair, and a Tarot deck. His teeth appear to have been filed to points, and people who have met him say he "laughs like a dead person."