He was a Forerunner, but then he mutated his own body and was cast out by his own kind, a scarring backstory strikingly similar to another guy no-one cares about. He was supreme commander of the Forerunner forces, and he had it all: Ships, Planets, Technology, and even the apparently beautiful Librarian was merely putty in his hands. But then came the Flood and the Reapers. They destroyed the Forerunner civilization and as each planet fell, the Didact became more and more desperate. In a final act of madness he decided to completely kill off everything in the universe, except the Flood and Reapers. Good Job.
He then became very lazy and built machines to do work for him. Weird floaty powers to pick things up, clothes that put themselves on over him, even a big orange ball to fly around in. As the Librarian watched with increasing disdain the Didact slowly became insane.Johnny, and the Didact became mad. He mutated his body to make himself awesome but he was then outcasted by the Forerunners and left to prowl Requiem for all eternity. Even Jorgey bullied him into giving up his best Composer, and Didact got stuck with the crappy one.
He was still desperate to Compose all of Humankind so he fled Requiem and went to Earth. There, he composed one city and laughed to himself 'Mwa ha ha, i have composed less than 0.1% of the Human population! Aren't I awesome?' To his dismay, it turns out he wasn't awesome and his ship got bullied by the Infinity.
In the end, Johnny put him out of his misery by blowing him up in the charred remains of all that was left to him. How nice is that, eh?