I read a book series a long time ago that I remember parts of, but I can't find via Google. It is in the late period of the Roman empire, after it has split in 2. The main character is from the Eastern half. He starts a a young man or youth, his family is powerful. He is exploring magic, and thinks that the entire empire is controlled by an ancient magic.
There's also a different guy who I think is Persian, who is a sorcerer who accidentally opens a door to another world, and uses power from there, but is also constantly holding it shut via magic from super powerful beings who want to destroy the world.
At one point, a woman who is with the Roman character thinks that he is becoming evil and stabs him in the head, but he's so powerful at that point that he stops his death, repairs the damage.
He finally takes control of the magic that is controlling the empire and basically takes over everything.