The Green Mist is the main antagonist in the 2010 Narnia movie, The Voyage of the Dawn Treader, although it did not appear in the book.
The mist came from the Dark Isle and was a formless substance that could enter into one's deepest, darkest thoughts and make them real. It required sacrifices from the pirates of the Lone Islands, who fed it boatloads of living humans. The only way that it could be defeated was if the Seven Swords were laid at Aslan's table. It was responsible for the creation of a huge, terrifying sea serpent, via Edmund's thoughts. Also from Edmund's thoughts came the resurrected memory of the White Witch.
The Green Mist was finally defeated when the sea serpent was killed and the Seven Swords were laid at Aslan's table. All of the people whom it had "eaten" were released.
Some fans speculate that the Green Mist is somehow connected to the Lady of the Green Kirtle, the main villainess from the subsequent book, The Silver Chair. It was confirmed in the film's commentary that the Lady of the Green Kirtle was behind the Green Mist's existence. However it is unknown if she is also responsible for the existence of Dark Island or if she merely enchanted Dark Island to spawn the Green Mist.
The Voyage of the Dawn Treader
The Silver Chair
The Horse and his Boy