Nearly one thousand years ago, the Sunderking led a rebellion against the empire of yuan-ti that dominated the realm, the Ssthessic Vrasa. They had killed his wife and daughter, and he was driven by vengeance and desperation. But he was a skilled warrior, and under his leadership the slaves of Shallowport were freed and the Ssthessic were put to death. 

Over the following decades, he waged a war of total annihilation against the Ssthessic. He freed all of his people from their grasp, burned their cities to the ground, and slaughtered them wherever he found them.

When he heard that the Ssthessic had an oracle that could see the future, he pursued her to Tol Fallent and killed her, drinking her blood in a ritual to give him his own vision. What he saw only drove him even more mad. 

He made a pact with a dark god to sunstain himself into undeath, and ruled with an iron first until 187A.S. Then he was killed - or so we thought. 

His pact had included a way to bring him back to life, using his body, his weapons and armour, and an enchanted stone. They were scattered throughout the Kingdom, but the knowledge that he could return persisted as a legend. They called him the King That Was, and King Would Be Again.