The Idiot was created when a man named Dr. Crosby sought to cure four men with damaged minds by telepathically linking them together with the use of a hallucinogenic Amazonian plant known as the Idiot Root. It was his belief that the four men could together create a mental world of their own, in which they could then gradually heal their minds. The experiment proved to have an entirely unforseen side effect however. Dr. Crosby came to discover that the subconscious minds of the four men had somehow merged and created a fifth personality, a demented mental entity that came to call itself "The Idiot."
Idiot was created by Peter Milligan and Norm Breyfogle. He first appeared in 1991's Batman #472, after which his story then hopped back and forth between Batman and Detective Comics, so that part 1 was set in Batman #472, part 2 in Detective Comics #639, part 3 in Batman #473, and the final part in Detective Comics #640