Ur Didact was the original Didact that had lead the forerunner warriors for thousands of years. He is the one in halo 4. ISO Didact is a former forerunner manipular (12 years old initially) who had the ur didact's consciousness imprinted upon him. This consciousness grew and took over the original, becoming a new Didact (ur means original. ISO means new) ISO Didact lead the forerunners in the originals stead.