Sithoaag was the Dark God of Madness, Uncontrolled emotions, and Music. He appears in the form of an emaciated (nearly skeletal), elderly man with wrinkles, stringy, dead white hair, pitless white eyes, and his skin color pale as snow. He was frequently associated with madmen, psychopaths, serial killers, and musicians. He was responsible for all mental deterioration of mortals. Sithoaag met his material demise at the hands of another god, his daughter in fact, the monstrous god called Slaanesh , whom devoured Sithoaag.
Sithoaag frequently appeared with a golden mask that depicts three side-by-side faces, each expressing a different emotion; Anger, Joy, and Sadness.
Sithoaagsided with Aku during the Aetherium War against Auriel and the Divines. After the defeat of the Darks, he retreated to his pocket of existence, the realm known as The Asylum. There, he laid in wait until the Black Diamond War, where he and several Dark Gods freed the fallen Brother of Order Sargeras' soul fragment, and used their power to create a new plane. Here he erected a massive temple to himself in this new universe that came to be known as Klendathu.
Sithoaag was also spoken to by the Divine goddess Nibelle years later. She attempted to seduce him and convince him to join the Divines. Sithoaag was momentarily confused, but the conversation turned into intercourse rather quickly. The goddess Nibelle was made pregnant, and bore the child of Sithoaag. She was forced into hiding however, as such promiscuous behavior was forbidden by the code of the Divines. Forced to live in The Asylum, she grew to resent the father of her child, and being away from the light of Aetherius darkened her mind, and allowed her mind to become fouled, just as the Dark God's seed had befouled her womb.
Eventually, calling Sithoaag to her side, she gave birth in extreme pain, as the dark creature inside her clawed for freedom. The birth did not go as planned - the child ripped through her mother's belly, while she was alive. Both gods were alarmed by what came forth, a foul pale goat-like creature with razor sharp teeth. The child let forth a bloodcurdling screech before consuming her mother and her father alive. This creature, a new god, came to be called Slaanesh.
Sithoaag's true personality is unknown because he is utterly unpredictable. His very nature is bipolar, and can shift from happy to furious to sad nearly instantaneously. He is just as likely to help or kill those whom he encounters. He is known to make tempting pacts with mortals, only to trick them into going mad. However, he seems to be quite loyal to Aku. This could be because Aku has no problem with Sithoaag's madness, whereas Auriel may find it dangerous. When he appears to mortals, he often enjoys speaking in rhyme and riddle to complicate things for his "clients".
It is believed that Sithoaag created music, using the bones and tendons of other gods he killed during the War. He made drums with skulls and bones, and he created a guitar with the ribs and tendons of dead Nu'ada. It is said he played his intruments while Aku's forces fought the Divines.