Arawn was an evil overlord in the Chronicles of Prydain books by Lloydd Alexander. He was replaced by the Horned King in The Black Cauldron because the animators felt that people would like the Horned King better since he had horns (similar to Maleficent and Chernabog). However, Arawn did recieve a part in the movie. During the prologue, the narrator talks about the history of the Black Cauldron and how Arawn came to become it, Arawn (as the cauldron).
As stated in the prologue: "Legend has it, in the mystic land of Prydain, there was once a king so cruel and so evil, that even the gods feared him. Since no prison could hold him, he was thrown alive in to a crucible of molten iron. Ther his demonic spirit was captured in the form of a great Black Cauldron."
Arawn's face is apparently engraved upon the cauldron. At certain points in the movie, the face is illuminated.
In the House of mouse episode, House of Magic, Daisy Duck accidentally makes the House of Mouse audience disappear. When they ask the Magic Mirror for help, he tells them to check the prop basement for something magical to help them. Daisy and Minnie find the Black Cauldron (a sign on it says: Who So-Ever Uses the Black Cauldron for Evil Will be All Powerful), and decide it's what they need.
Daisy dresses as Maleficent and attempts to cast a spell. She ends up summoning the Cauldron Born instead. In order to make it go away, she casts a spell. However, in the process, she makes the whole House of Mouse disappear. After everything was set straight by Jafar, Daisy attempts to use the Cauldron as a fondu pot, again summoning the Cauldron Born.
Behind the scenes
Arawn was originally to be the main villain of the Black Cauldron, just like in the books. However, Arawn was replaced by the Horned King because animators felt that audiences would like Horned King better because he had horns. Several people working on the film were discouraged about this because they had read the books and felt that you couldn't do the film based on the book without the book's main villain. The film workers urged the producers to give Arawn a part in the movie so they made him a spirit trapped in the cauldron.
Role in the series
Originally, Arawn was Chernabog's finest warlord, leading the forces of evil to battle. So when Arawn was locked away in the cauldron, Chernabog decided to make the cauldron a portal to Hell where his forces of Darkspawn where residing just because there wasn't enough portals for his minions to escape into the real world. The rest of the cauldron's bio can be seen on it's page.
The Cauldron will soon return in Spirited Away, having been stolen from the Witches of Morva by Master Xehanort's Shade, and will soon be used to bring forth dark spirits from the graves of Japan and Prydain to seek their revenge, only to end up turning the story into a ripoff of the final act from KH 3D. And who knows? Maybe Arawn might make an appearance in the film as a darkspawn spirit himself.
An old servant of Arawn, the Wishing Skull, will soon appear to threaten the life of Ignitus and the Shell Louge Squad in a future chronicle movisode to come.