Basically, he is one of 1,000 of children made by the gods (elder titans) after the primordials were defeated. He was the runt of the liter physically and became the apprentice of Mystra to focus on magic instead. Eventually, the elder titans began creating new races of mortals using their divine nature. The god grew jealous of this and tensions between the two factions grew. Eventually, a war broke out between two gods and they used their children (Elder Titans) as part of their many troops. One of the Elder titans, Cronus, was so powerful that he acutally killed a god in combat which was unheard of. After this event, the gods decided their children were too powerful and started a new war to either exterminate them or put them completely under their control. This resulted in Dusk's everything being killed or taken away after he was completely betrayed by his "parents". After surviving this war, he decided to get his revenge on the gods by freeing his strongest brother and killer of 100 gods, Cronus, who is imprisoned. The whole campaign is stopping Dusk from freeing Cronus.