The Demon Lord Belial is a DLC for Darksiders II.
The leader of Earth's stranded angels Uriel has called Death back to the Earth to share with him some important news, recently her men have reported sightings of what just might be the last of the humans and asks the Horseman to investigate.
Arriving at whats left of the human race, Death is aided in the fight against several demons by The Hunter whom claims to be the last of humanity much to his shame. He reluctantly explains that a demon lord named Belial has taken custody of his soul and lest the Hunter does what he says then it will be forever damned in Hell. To this end the Hunter has been responsible for the deaths of the rest of his kind by leading the demons to them.
Death decides to hunt down Belial at the Hunter's request, once in the heart of his territory Death and the Demon Lord exchange banter. Belial explains that he holds no power over the Hunter's soul, the mere threat was enough to buy his compliance, and doesn't consider the "measly human" a threat. With the banter finished the two enter a grueling fight in which Death is victorious.
Death returns to the initially relieved Hunter only to inform the now-mortified human that Belial never held any power over his soul, overwhelmed by the grief the Hunter makes one final request of Death, his death to which the Horseman reluctantly complies.