Randall Flagg (also known as Walter o'Dim, The Man In Black, Marten Broadcloak [court magician to Roland's father Steven] and many others) is a man with extraordinary magical powers. He has been walking the Earth (mainly Mid-World, but he can go to most places and eras) for a very, very, very long time. He is the Crimson King's second in command. His original name is probably Walter Padick, son of Sam.
He certainly is a very interesting character, one of the most interesting in the Dark Tower series. He has a character-arc of his own that is compelling, if only because we only glimpse it in small flashes.
He is, according to Stephen King, the worst in us all, the embodiment of pure evil, and I respect that.
