The Bloody Path to Apotheosis
Taked directly from the Forgotten Realms Wiki page:
As one of Mystra’s Chosen, Sammaster possessed great power. In addition to his own skills he also possessed a fraction of Mystra’s essence like Elminster and Khelben “Blackstaff” Arunsun. In Sammaster’s case, however, the additional power brought delusions of power and madness, which consumed him and triggered a series of events that, in the end, cost him his life. With help he rose from the dead and sought out knowledge. He came to believe in time that “dead dragons shall rule the world entirely” and spent his undeath working towards the realization of his goal. Often he has crossed paths with the Chosen of Mystra and even the god Lathander in his quest.
In 1373 DR, he bound his phylactery to the Dracorage mythal, causing it to no longer have a connection with the King-Killer Star, starting the Rage of Dragons, which effectively caused all dragons on Faerûn to go mad. His motivation was to turn all chromatic dragons into dracoliches without interference from the metallic dragons. His plan was eventually foiled by Dorn Graybrook and his group of dragon hunters, Brimstone, Karasendrieth, Tamarand, Nexus, The Talons of Justice, and a group of Thentian Mages.