Sargeras is the strongest of the Titans. He was created to resist the great evil and clear the Universe from demons of Twisting Nether. But once he realized that his real enemies had been Void Lords, not mere demons, faith shattered. Void Lords were the creatures consisting of the dark matter and devouring all the substances and energies of material world. Sargeras came to conclusion that death is the only way to save the Universe from such a threat. Everything living must be dead. The Titans were indignant with such ambitions and refused to help Sargeras on his new path.
Sargeras set free the demons of Twisting Nether and let them out of Mardum, a prison, created by himself. On pain of death the demons agreed to serve Sargeras; Mardum was destroyed, and Fel Energy that once overwhelmed the great prison became a part of Sargeras’s consciousness, mind, and his very essence. That is how Burning Legion with countless horde of demons came to be. The Titans were destroyed, and Sargeras directed his sight at Azeroth, where a new titan, even more powerful than Sargeras himself, must had awaken.
The power of Burning Legion still was not absolute. The demons were too foolish to cope with everything on their own. Sargeras craved for new allies, wise and insidious commanders, who would be able to rule the army of cruel bloodthirsty demons. It happened so, that the Eredar, a highly developed race from the planet Argus, turned into such allies. Kil’jaeden and Archimonde were the strongest of the Eredar. Sargeras managed to corrupt their minds and convince the people of Argus to join him, promising to turn the planet into a hub of peace and intellectual development. He also promised to share the deepest secrets of the universe and its endless knowledge. Sargeras then filled Kil’jaeden and Archimonde with Fel Energy, and doing so converted them into his most faithful servants. Sargeras was finally prepared to start the great war. And Azeroth torn apart by pride of the elven queen Azshara, was not ready to resist the deadly threat.