Coriakin was a wizard and originally a star, charged by Aslan to rule the Duffers and guide them to wisdom because of past misdeeds, the exact nature of which was never known by humans (a fellow star, Ramandu, mentioned they were not for human beings to know).
During Caspian's journeys on the Dawn Treader, Coriakin appeared as a stereotypical wizard, complete with a beard and ornate robes, with one additional detail: he always walked barefoot. He lived in a mansion that was reminiscent of English mansions on the island. Inside his mansion were items such as a spellbook, and other odd items, such as a bearded mirror, which was a little mirror with hair and a beard attached to it.
Though the Duffers saw him as evil, he was actually kind and wise, treating them with love, though sometimes annoyed by their antics. Some details also imply that he had a quirky sense of humor, like the fact that he turned the Duffers into Monopods for disobedience, and the bearded mirror which could have been used to prank his guests.
Governing the Duffers
To govern the Duffers, he required the use of magic, although they will be ruled by wisdom in the distant future. Because of the dim-witted stubbornness of his subjects, Coriakin cast a spell that merged each Duffer's two legs into 1 leg. The Duffers, believing themselves to have been "uglified," snuck into his rooms and cast a spell to make themselves invisible, hiding their new appearance. The spell also affects Coriakin.
The Visit by the The Dawn Treader
The Duffers eventually grew tired of being invisible, and, when The Dawn Treader visited their island, coerced Lucy Pevensie into reading the spell that would make them visible again. The spell could only be read by a young girl, and the Duffers were too cowardly to send any of their own daughters. After perusing through a few other distracting spells, Lucy made all things visible in the magician's house (including Coriakin and Aslan himself).
Lucy soon saw the magician as a kind, wise fellow, hardly the terrifying sorcerer the Duffers made him out to be. He suggested that she try to convince the Duffers that their new appearance is nicer than their former one, which she succeeded in doing. The Duffers renamed themselves Monopods, but kept mixing it up with their old name and came to be known as Dufflepuds.
Dufflepuds are among the creatures that entered Aslan's Country after the final battle in Narnia.
In the movie, six of the lost Seven Lost Lords found their way to Coriakin's Island, and asked him about The Green Mist that was threatening their world. Coriakin knew about the danger, and told them to go to Ramandu's Island.
Because of the dire threat the Mist represented, Coriakin himself cast the spell of invisibility, which made him, his home, and the Duffers all invisible. He said it seemed the easiest way to protect them from the evil, although the Duffers highly disliked this, claiming that he had oppressed them. When the crew of the Dawn Treader arrived, the Duffers used Lucy to go into Coriakin's mansion, to cast the spell that would make them be seen again. Although it came at a rotten time, as they were fighting the crew, and were convincing them they were vicious and terrifying monsters, which seemed to be working...until the spell wore off, and they all saw them for what they truly were.
In the film, Coriakin appeared as a gentle and mild-mannered man. Definitely not the oppressing kind, as the Duffers believed him to be.
Coriakin then told the crew about the Lords who had passed by his island, and who he had sent to Ramandu's Island. He informed them about the Seven Swords, and how placing them all on Aslan's Table was the only way of destroying the Mist. He also warned them to be wary, as until they did that, evil had the upper hand, and that it would forever try to tempt each one of them.
Nothing was mentioned in the film about his past, or that he was even once a star.