Originally Posted by
W1shm4ster
Yep, i was wrong the Nathrezim(dreadlords) were the "cause" of his corruption, at least of his final lose of faith in the right.
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. While his confusion and misery deepened, Sargeras was forced to contend with another group intent on disrupting the titans' order: the Nathrezim. This dark race of vampiric demons (also known as dreadlords) conquered a number of populated worlds by possessing their inhabitants and turning them to the shadow. The nefarious, scheming dreadlords 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.
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.