"I should have known you'd risk your life to save humanity, just as the Librarian died trying to save you... And now, I'm going to do what I should have done all those millenia ago!"
-The Didact before attempting to kill Lucario
The Didact is an ancient Forerunner, a member of a nearly extinct race of extremely advanced aliens. He was once the general of the entire forerunner military, and he has a reputation for hating humanity. This hate was born primarily of jealousy of humanity's favor in the eyes of his fellow forerunners as well as the Gods. He upheld an old belief of the forerunners known as the Mantle, which stated that they were the sole guardians of all that existed. However, when favor shifted from them to humanity, the Didact used the Mantle against humanity, and waged a war against them. The war ended in old humanity's destruction, however, his fellow forerunners had him imprisoned in a cryptum for millenia on end.
The Didact rose again in 2100's, during the time of Blue team. He was discovered on an ancient Forerunner World by Covenant separatists under Jul 'Mdama, and he was instantly recognized as a living god. He used this to his advantage, and, using the Covenant, waged war on humanity once again. This time, he met his end during the final siege of Earth. He intended to fire a death ray upon the planet to exterminate all humans, but he was "killed" by Lord Vilgax before he could fire it. His body was later flung out into space, forever lost (possibly).