To begin with Khull Khumm was a faithful and nice sort, but became power hungry. Over a millennia ago, Atlantis was at war with Ys. Khull Khumm granted Ys the power they needed to destroy their rival. This resulted in the apocalyptic Devastation that destroyed both great lands. He proceeded to harness power from the mass deaths and entrapped the Younger gods into orbs. Thera managed to prevent the Shadowking from having his way by floating the orbs out of his grasp.
Sensing the presence of magick and the orbs, Khull Khumm set his sight on the castle of Stonekeep and cast a dark magic that killed the inhabitants of the castle and reduced them to bones and then caused the castle to sink several feet underground. Drake was the only survivor as Wahooka had managed to teleport him to a safe place to train for his destiny.
Ten years later, Drake (in semi-spirit form) ventures with Thera into Stonekeep, but they are shortly confronted by Khull Khumm, who ridicules Thera's poor choice of a companion to challenge him. The Dark God then shortly entraps Thera in her Earth Orb, mocks Drake's futile efforts to thwart his goal and implies they will meet again.
As Drake explores Stonekeep and the realms below, he finds some of the minions are working for Khull Khumm including the Throgs and the Ice Queen. From time to time, Drake comes across ab orb that resembles one of the Solar System planets and imprisoning one of the lost gods. Those orbs are what the Shadowking seeks to gain absolute power. Drake learns about Khull Khumm's true goals from the Faerie Queen Iaenni as well as the history of the devastation.
Khull Khumm confronts Drake and his party, while they are progressing through the Palace of Shadows. He gloats at them and gives them a mighty push, causing them nasty injuries and disappears.
The final confrontation with Khull Khumm comes at the top of his tower. He tries to convince Drake to hand him the orbs as he cannot take them directly. When the god of Pluto Kor-Soggoth intervenes, the Shadowkings threatens great pain on Drake if he does not hand over the orbs. If Khull-Khumm does get the orbs, he will eliminate Drake and his companions, seize all their powers and destroy Thera's temple.
There are three ways Khull Khumm can be defeated. The first is to trap him in an orb of his own with four silver objects being placed on the pedestals surrounding the spot where the Shadowking stands. The second is to slay the Shadowking with the Shadow Blade or Black Slayer Arrows. The third is to retreat to Thera's temple, free the gods and let them take care of their Dark Brother. In the even of battling Khull Khumm, it is revealed that he fears Wahooka who holds more power than he could muster.
According to a recipe at the end of the game credits his favourite food is Chocolate Chip Pumpkin Muffins.