Mad King Leoric
Available at the start of the quest: A Shattered Crown
Disappears at the start of Sword of the Stranger
Leoric's madness was caused by Diablo?
Leoric resisted Diablo's attempts to posses him, but the effort left him weak and confused. When his son Albrecht was kidnapped to be a vessel for Diablo, it destroyed any sanity Leoric had left.
After Leoric was Skeleton King, what then?
He commanded an army of the dead deep within the cathedral, finally serving the Lord of Terror he had resisted while alive.
Many adventurers tried and failed to slay the Skeleton King. It was left to Leoric's eldest son, Aidan, to finally kill the monster his father had become.