Diagon is believed to have visited Earth many years ago and gave humans advanced technology. He is also believed to have sworn to come back to Earth to bring the "Golden Age". He claimed he would bring alien technology so that Earth will not have war, nor disease, just peace. Vilgax was mistaken for, and pretended to be Diagon in The Flame Keepers' Circle.
Diagon is in fact an extra-dimensional demonic entity who tried to invade this dimension with the help of his Lucubra army during Earth's medieval era. He was banished back to his dimension when Sir George defeated him by cutting out his heart with Ascalon, a powerful weapon given to him by Azmuth nearly one thousand years ago.