Kool Keith, aka Black Elvis, Dr. Octagon, Poppa Large, Dr. Dooom, and about a dozen personae, got his start with the Bronx-based Ultramagnetic MC’s in the mid-1980s. Although revered, their recordings made little commercial impact and the group disbanded. Since then Keith has enigmatically swung from project to project, maintaining a deliberately inscrutable cloak of aliases and alter …