Heart of the Swarm (details, discussion, etc) - Page 16

"Sargeras has changed over the millennia. While he once served his race as guardian and protector, he is now the titans' worst enemy. Sargeras works to undo what the titans have strived for ages to achieve. He is now the master of all that is evil and demonic. Lesser beings quake and flee in his presence"

"Shaken by the evil of demons in general and the nathrezim in particular, Sargeras began to despair of his task and gradually slipped into a brooding depression. As doubt and despair overwhelmed Sargeras's senses, he lost all faith not only in his mission, but also in the titans' vision of an ordered universe. Eventually, he came to believe that the concept of order itself was folly, and that chaos and depravity were the only absolutes within the dark, lonely universe. His fellow titans tried to make Sargeras realize his erroneous way of thinking and calm his raging emotions, but he disregarded their more optimistic beliefs as self-serving delusions. Storming from their ranks forever, Sargeras set out to find his own place in the universe. Although the Pantheon was sorrowful at his departure, the titans could never have predicted just how far their lost brother would go"

Sargeras roamed the Twisting Nether, freeing the many demon races that he had previously hunted down and imprisoned. He became known as the "great, dark god of the Nameless Void". He bound the newly released demons to his will and used them to construct an army of massive proportions — one that eventually became known as the Burning Legion. These cunning creatures bowed before the dark titan's vast rage and offered to serve him in whatever malicious ways they could. However, among the various beings who served him, none of them had the natural cunning and power to act as commanders and tacticians for his new army, so Sargeras looked about for a race who would serve as his favored agents in exchange for horrific power

"On the planet Argus, Sargeras found the agents he sought — the eredar, a race of highly intelligent beings with a natural affinity for magic in all its myriad forms. Sargeras appeared to the eredar's three greatest leaders, namely Kil'jaeden, Archimonde, and Velen, and made his offer: untold power and prestige in exchange for their allegiance. Though Velen had doubts, Kil'jaeden and Archimonde readily agreed. Velen and his followers fled, becoming the draenei. The remainder of the eredar embraced the darkness and became the stuff of nightmares: the man'ari eredar."

"Magus or Magna ,Medivh was the last Guardian of Tirisfal, an ancient line of protectors bestowed with great powers to do battle with the agents of the Burning Legion. He was corrupted even before birth, and possessed by the spirit of the demonlord Sargeras who used Medivh in his plan to conquer Azeroth"

Shortly before the beginning of the Third War, Medivh returned as a mysterious prophet, manipulating events in order to arrange the formation of a last-ditch alliance between the mortal races on Kalimdor, guiding them to victory at the Battle of Mount Hyjal The prophet's name means "keeper of secrets" in Thalassian"