Dark Kahn is the final boss in Mortal Kombat vs. DC Universe. He is the driving force behind the Rage that is affecting warriors from both worlds.
Not much is known about Dark Kahn other than he is the result of the fusion of Shao Kahn and Darkseid's essences. When Shao Kahn was hit by Raiden's lightning while trying to enter a portal, Darkseid was also hit by Superman's heat vision while trying to enter a boom tube. The two events both happened in the same space and time of their respective universes, and the ending result was the merger of the two universes and the creation of Dark Kahn.