Ha, Ha, Ha, Ha...
The Skeleton King was the final guardian spirit guarding the exit out of Dimension of Death. He is one of the Skeleton Commanders.
Background
He was in charge of a contingent of Skeletons. He claimed to be in charge of a new order and intended for it to prevail.
He called for Connor to come closer so he would learn of his imminent death. But Connor had no desire to die by the spirit's hand, as many a corpse would attest. The commander intended for his quest to end at the Bridge of Life, for no mortal crosses the River of Death. Connor's quest was to restore the mask of eternity to where it belonged, and would not let the commander stop him. So the commander told him he would meet his death, and he wouldn't even need to soil his hands with his blood doing it. He then ordered his minions to attack Connor. Laughing maniacally as his soldiers marched towards Connor.
Connor quickly dispatched his minions. The commander then thought Connor may prove be a worthy opponent after all, and that he was ready for him. Connor then went after the commander himself. The commander told Conner to be prepared to die, and that none shall leave the Dimension of Death. Conner replied, "We Shall See". Conner defeated the Commander, and took his Double-edged Sword.
Behind the scenes
This character is known as the Skelking (Skeleton King) according to the debug mode, and within the programmer notes in 2000.msg file.