The King of Hell is the ruler of devils / demons, including being master to Demon Lords such as Mordja and Nahaz. Long before the creation of the world (possibly the universe), UL bound the King of Hell in chains that bound him to Hell. (Hell appears to be another ... universe or dimension, as mentioned briefly in the Seeress of Kell.)
He is a hidden antagonist in the Malloreon until the very end. The King of Hell sought to use either Mordja or Nahaz to gain the Sardion and use the Sardion's power to wrest the Orb of Aldur from Garion. Then he would use the power of both stones to become the new God, and break his chains and then contend with UL.