The Chaos King is a major supporting antagonist in the Kingdom Hearts Unlimited Saga, only appearing in "Encoded Truths" as one of the main villains of that installment.
A powerful Darkheart Lord-Digital Mutate hybrid born from all the digitized entries of every single Darkheart faced by the Highwind Crew during their adventures up to that point, the Chaos King was created by the Lord of Darkness using Abraxas' viral-hacker abilities and his own dark powers to serve as his emissary and bringer of destruction in order to spread havoc across the worlds connected to the Datascape/Digital World and use them as the means of bringing about his return to power in the United Universes. The Chaos King is also responsible for corrupting the research data compiled by Morgause and stored within Count Dooku's mainframe server into Bug Blox to serve as its means of in turn corrupting the Digital World, knowingly passing down the secret of their creation to Queen Grimhilde and her incarnation of the Hellfire Organization so that he could manipulate them into unwittingly doing his master's bidding. It is therefore up to Emma Swan, Jim Hawkins, and the rest of the Gang back at Disney Castle to stop the Lord of Darkness and Chaos King's machinations before they are able to escape into the Real World.
Later on, it is revealed that the Chaos King is secretly plotting to backstab Lord of Darkness and take full control over all the Bug Blox and Darkhearts so as to ascend into godhood and continue Chernabog's mission of bringing chaos and misery to the universe, feasting on the hearts of mortals who fail to escape his grasp. To enact his treacherous plans, the Chaos King delves deep within the mainframe servers he corrupted and discovers the entry compiling the period of time when Taran sacrificed himself by removing his heart-soul to save Princess Eilonwy from the Sleeping Death curse. Believing that if he were to enfuse this data entry within his being, he would be able to gain all the powers of Taran, including the ability to wield a Keyblade, the Chaos King pulls out all the stops to ensure he gets what he wants in the end, regardless of the massive amount of suffering he has to inflict on the heroes trying to stop him.