Leoric was the king who ruled over the lands of Khanduras, apparently after annexing it from the previously feuding kingdoms. He was widely known as a noble and just king who, like many before him, ultimately fell prey to the influential corruption of the Lord of Terror, Diablo.
Not much is known about Leoric's origin's other than that he came from the east as an "Eastern Lord" in the company of an Archbishop of the Zakarum, Lazarus, and other servants of the faith. Leoric was a stout believer in the Zakarum tenets and though Lazarus was Leoric's trusted advisor it was by his decision that Leoric established his capital in Tristram, not that of the future king himself.
Reign in Khanduras
Leoric was a foreign king but the people quickly acquired respect for his just rule. He brought order to the land, but it was not long before problems began to arise. Unbeknownst to Leoric, the old monastery which he had chosen placed his seat of power in was the old Horadrim Cathedral where Diablo had been buried almost 240 years prior, and soon after arriving Lazarus found the soulstone below the cathedral and smashed it. Upon his release Diablo gazed upon the town above and judged Leoric to be the individual with the strongest mind above, and thus the most fitting host. The Lord of Terror set to slowly corrupt the king's mind, and Lazarus returned to the surface to aid him in this endeavor.
Leoric senses this strange presence penetrating his mind, but he fears what will happen if his priests find out that something is trying to control him. Believing himself to be devout enough to exorcise, he tells no one what is happening. For five years Diablo works himself into the mind of Leoric, slowly corrupting him, and the king slowly loses the battle.
The king starts to adopt a much more violent reign, forcing villagers and townspeople into submission by military might, and his citizens begin to call him the Black King. As Leoric's state deteriorates, his advisors, friends and family start to suspect that something isn't quite right with him. Lazarus fears what will happen if they find out that Diablo is trying to possess him before the process is completed, and so convinces the king that Westmarch, the neighboring country, is planning to go to war with Khanduras. Leoric flies into a rage and sends off the army towards Westmarch despite loud protests from his other advisors.
With the prying court mostly gone with the army, Diablo tries to complete his hold over the king, but much to his dismay finds that the king's spirit still lingers and resists Diablo. Realizing that he can never corrupt Leoric fully, he relinquishes his hold over the king. Leoric is left crazed, and when Lazarus secretly abducts his son Albrecht and drags him into the catacombs beneath the cathedral, Leoric loses what grip of sanity he has left. He accuses the townspeople of conspiring against him and executes many of them, including his own wife and queen Asylla. When Lachdanan, the captain of his knights, returns from the disastrous war with Westmarch and sees how maddened the king has grown he kills him. As Leoric dies he curses all of his former knights.
Rise as the Skeleton King
Though Lachdanan has killed his king, he deeply regretted what he had to do. He and his remaining knights attempted to bury the king below the cathedral. As they entered the burial chamber, however, Leoric rose from the dead and attacked what knights were left. Only Lachdanan managed to escape the encounter, and Leoric, now called the Skeleton King, began to raise the fallen knights as skeletons to serve him. Not long thereafter his eldest son Aidan would return to Tristram and slay his father.
The Skeleton King is the first boss encounter in Diablo III and the final encounter of the Diablo III Beta.
"From the east shall come a monarch whose rule begins in blood and ends in bone. In the light of a comet he will die a third and final death at the hands of <true men/a chosen woman>."