@mcmonkeyplc The Didact is the head of the Forerunner's military. He defeated the original human army, and had them genetically regressed as punishment. When they found we had been fighting the Flood, not them, he assembles a new army - by using the Composer to reduce us to atoms, and reassembling said atoms as the Prometheans. He kept a bit of the human form in them as a mockery of his former enemy, us. The Librarian then locks him up in the Cryptum, for his crimes, and brings his army under her control to guard him.
So he created the Prometheans to fight the flood?