Netheril was at war against the phaerimm and desperate trying to survive their enemies' life drain dweomers. A great archwizard that was working to solve it disappeared, causing panic to surge in Netheril. Karsus, a very powerful wizard, felt the weight of responsibility on his shoulders. To gather the power needed to preserve his civilization he made a terrible decision. He used the powerful spell Karsus' avatar, that allowed its user to steal the powers of a god of his choosing, thus replacing the god. He used it to replace Mystryl, the goddess of magic, and as her power was drained she became unable to maintain the Weave, having to sacrifice herself to save it. Magic stopped working, Netheril fell, and Karsus was turned to stone, but soon Mystryl was reincarnated as Mystra and fixed the Weave.
I think the Absolute is the disappeared archwizard. His name was Ioulaum, and he was the inventor of the mythallar and the flying fortresses. Since the fall of Netheril he became a lich, created a lair in the underdark, had many apprentices including illithids, created alhoons (limited illithiliches) and illithiliches. Later he was slain by his illithilich students, but that was a ritual he designed to create an undead elder brain that he would eventually merge with. He accomplished what he wanted and became the Oracle of Ellyn'taal, the undead elder brain. He's probably one of the most powerful beings existing in Toril.
