He was released after the forerunners captured the last human world and signaled their defeat. In a Colosseum of some sort the didact approaches a time lock capsule where the forerunner began unlocking enough levels to be able to talk to the prisoner. Unfortunately it was found after the halo rings were fired that it destroyed precursor tech. It turns out some of the time locks, the original prison was precursor and was thusly destroyed as well. That's my answer.