Who is Sargeras
Sargeras was the great champion of the Titans, chosen by their Pantheon to defend the worlds they had created. He carried out these duties for countless millennia, seeking out and destroying the demons of the Twisting Nether wherever he could find them.
The Eredar, an insidious race of devilish sorcerers, used their warlock magic to invade and enslave a number of worlds. The indigenous races of those worlds were mutated by the Eredar's malevolent powers and turned into demons themselves. Though Sargeras' nearly limitless powers were more than enough to defeat the vile Eredar, he was greatly troubled by the creatures' corruption and all-consuming evil. Incapable of fathoming such depravity, the great Titan began to slip into a brooding depression.
As doubt and despair overwhelmed Sargeras' 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 persuade him of his error 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.
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.
From the ranks of the powerful Eredar, Sargeras picked two champions to command his demonic army of destruction. Kil'jaeden the Deceiver was chosen to seek out the darkest races in the universe and recruit them into Sargeras' ranks. The second champion, Archimonde the Defiler, was chosen to lead Sargeras' vast armies into battle against any who might resist the Titan's will.
Once Sargeras saw that his armies were amassed and ready to follow his every command, he launched his raging forces into the vastness of the Great Dark. He referred to his growing army as the Burning Legion. To this date, it is still unclear how many worlds they consumed and burned on their unholy Burning Crusade across the universe.