Dark Kahn is a gestalt entity which exists halfway between the DC Universe and the Mortal Kombat universe that exists toward the end of the inter-dimensional invasion that occurred in Mortal Kombat 2. It serves as the final boss of the game Mortal Kombat vs. DC Universe.
It is a combination of the beings Darkseid, which was pushed into a portal by Superman after his invasion was defeated, and Shao Kahn, who was pushed into a portal by Raiden, possibly simultaneously. It may have been this single incident which caused the anomaly that the two universes' heroes and villains fight to stop throughout the game.
Dark Kahn reigns over the chaos of the anomaly in a combination of the lairs of Darkseid and Shao Kahn divided into halves bonded together by the same glow that emits during the rage anomalies that plague the beings of the game. It seems to feed on rage and on battle, so the rage effect probably only enhances its power.