Sargeras was a mighty giant of molten bronze who led the armies of the titans, the Champion of the Pantheon's cause. Like the rest of his kind, he was altruistic and just, unable to conceive of pure evil. Sargeras was called upon to defeat and imprison the hordes of demons native to the Twisting Nether, so that their evil would not contaminate the titans' vision of order. Sargeras went about his task devoutly for countless millennia.
During his never ending war against the evil in the universe, Sargeras became increasingly depressed by the chaos he saw wrought by all kinds of evil. Sargeras was forced to contend with the Nathrezim. A dark race of vampiric demons (also known as dreadlords) who turned whole nations against one another by manipulating them into blind hatred and mistrust. Sargeras defeated the nathrezim easily, but their corruption affected him deeply.
Sargeras began to despair of his task and gradually slipped into a brooding depression. 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.
By the time Sargeras' madness had consumed the last vestiges of his valiant spirit, he believed that the titans themselves were responsible for creation's failure. Deciding at last to undo their works throughout the universe, he resolved to form an unstoppable army that would set the physical universe aflame. 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. However non among 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.
Sargeras and the Legion eventually became aware of the young world of Azeroth due to the Night Elves’ reckless use of arcane magic, and Sargeras began to hunger for the limitless energies of the Well of Eternity. The elven Queen Azshara and her people, the Highborne, saw Sargeras as a god and worshiped him. Azshara agreed to allow him entrance to her world if only for a taste of a portion of his knowledge and energy. Thus, the first invasion of Azeroth began. Sargeras waited in the Twisting Nether for Azshara to strengthen the portal enough to allow him entrance. The Night elves, aided by Cenarius' woodfolk and Alexstrasza's dragons, mustered their forces and rebelled against the highborne in a climactic battle that destroyed the Well of Eternity and crushed much of the continent to rubble. Sargeras bore no smile as the portal he was going to use to enter Azeroth through was rapidly closing. Sargeras stepped into the midst of it, and while he was in the middle of the portal, it imploded and he was cast into a state of limbo.
Some nine thousand years after the War of the Ancients, Aegwynn kept a constant vigil over Azeroth, destroying demons wherever she found them. Sargeras saw that Aegwynn was a serious threat, but he also saw how she could be manipulated. Aegwynn found the rift from which the Legion's agents had come through in the Storm Peaks. Using this portal, Sargeras entered Azeroth in the form of an avatar. It is said that this avatar was infused with a portion of Sargeras' soul which he controlled. Aegwynn did not hesitate in attacking, but Sargeras held back, letting the mage destroy his physical body. However, as it died, Sargeras’ spirit reached inside Aegwynn’s body and hid, lying dormant for many years.
Years later, when Aegwynn's time as a Guardian was up and asked to choose her successor, she decided that only one of her own lineage would suffice. Seducing the mage Nielas Aran, eventually mothering a son by him. Eventually Sargeras’s spirit awoke and possessed the mind of Aegwynn’s unborn son, Medivh.
When Medivh reached fourteen, his powers as a Guardian manifested themselves throwing him into a coma that lasted for twenty years. During this time, Sargeras tapped into the Guardian's enormous power, and when Medivh at last awoke, the dark titan had nearly taken complete control. Sargeras contacted the orcish warlock Gul'dan, leader of a host of orcs on the world of Draenor. Medivh went to the Black Morass in the remote, southern parts of Azeroth and opened the Dark Portal allowing the orcs permanent access to the planet. Gul'dan's Orcish Horde swarmed through the rift, waging war upon the native humans of the Kingdom of Azeroth. Medivh's apprentice, Khadgar, helped the Human armies infiltrate Medivh's dark tower, Karazhan. Medivh was caught off his guard, and was in the end killed.
As Medivh died, the portion of Sargeras’ spirit that had possessed him was released and swiftly dispersed.This spirit was sent reeling in pain back into the Twisting Nether. To this day, it is unknown what really happened to Sargeras' real body or avatar. Many believe that Sargeras’ spirit yet endures, hungering for revenge against the only world ever to withstand the Burning Legion’s might...