Green Mist

 The Green Mist was a sort of mystical fog that was first seen in the East. There had been reports of fishermen and sailors disappearing out at sea, never to be seen again. The people of the Lone Islands were obviously the first to hear of it, and the first to suffer its power.

The exact time when the Mist first began appearing is unknown. But, apparently, with every person it took, the Mist grew steadily stronger. It preyed on people's weaknesses, their fears, and made their darkest dreams come true, to frighten or tempt them.

When the Seven Lost Lords from Narnia came to the Lone Islands, at the port of Narrowhaven, they learned of the Mist, and made a vow to find a way to destroy it. They each set sail, except for Lord Bern who was arrested and jailed for years, but none of them ever came back.

When more of their citizens began disappearing, the leaders at Narrowhaven became afraid, and in a bid to save themselves they began making sacrifices to the Mist, sending in people who they were unable to sell as slaves.

When the Dawn Treader arrived in Narrowhaven, Caspian X, Edmund, Lucy and Eustace were ambushed, while they were investigating the town, and put them in chains. The slave-trader Pug and his men were known to sacrifice slaves.

While Caspian and Edmund were jailed, they met Lord Bern, who told them about the Mist. Later that day, at a slave auction, when Drinian, Reepicheep, and the crew of the Dawn Treader attacked the slavers to free Caspian and his friends, Pug, the Governor Gumpas, and their men were killed.

When Caspian learned of the Mist, he vowed to not only find the missing Lords, but also put an end to it.

Along the journey of the Dawn Treader, they met the magician Coriakin. He warned them that the Mist was a danger to them all. He had even made the Duffelpuds (or Duffers) of his island invisible, to protect them, though they didn't see it this way.

Coriakin said that the Mist originated from a place called Dark Island, a place where evil lurked, and that it sought to corrupt all goodness, and steal light from the world. The only way to stop it and break its spell was by locating the mystical Seven Swords. These blades were originally a gift from Aslan, forged in the Golden Age of Narnia, meant to protect Narnia from evil. If they found all seven swords, and laid them at Aslan's Table, then the Mist would be undone. But until they laid down the seventh sword, the Mist would have the upper hand with them, forever trying to tempt them.

It tried to tempt everyone, but the ones it had the most effect on were Lucy, Caspian, Eustace and Edmund. With Caspian, it appeared in the form of his father, making him believe that he was a disgrace to him.

With Edmund and Eustace, it enhanced their avarice, making Edmund so full of greed that he was willing to fight Caspian. Eustace, too, was apparently affected by the Mist, when he found a Dragon's treasure, but did not know that the treasure was cursed to change someone into a dragon, should they try to steal it.

The more notable one was Lucy, as it enhanced her vanity, making her want to be as beautiful as her sister Susan. She went so far as to steal a page from Coriakin's book of spells, turning herself into Susan. However, the effects were not to her liking, and with Aslan's help she was able to overcome her jealousy.

Whether the Mist had a hand in what happened to each of the Lords is unclear, but when Lord Rhoop made it to Dark Island, the Mist kept him in a permanent state of intense fear, leaving him a broken man.

When the Dawn Treader arrived at Dark Island, the Mist took on the form of a giant Sea Serpent, and tried to destroy them all. It was only thanks to Eustace and Aslan, who changed Eustace back into a boy, and returned him to Ramandu's Island, that Eustace was able to place the last sword on Aslan's Table, thus ending the Mist's power for good.

Before he did, though, the Mist had taken on the forms on various people from the crew's subconsciousnesses. With Rhince, it took on the form of his missing wife. With Edmund, it took on the form of the White Witch Jadis, attempting to tempt him to join her, as the real White Witch did in life, but Edmund was able to reject her once Eustace laid the last sword down.

Appearances

 * Voyage of the Dawn Treader (Film)

Trivia

 * When Eustace tried place the sword on the table, the Mist attempted to stop him, apparently taking on a more solid form and grasping him in its clutches. This seems that the Mist can physically attack a person, should the need arise for it, though whether it did this regularly is unknown.
 * It was revealed in the DVD commentary that the power behind the Green Mist was none other than The Lady of the Green Kirtle. The producers/directors mentioned that they needed a villain, and that it was never stated in the Silver Chair book whatever happened to all the missing people, who went looking for the prince. Plus, she had to have been bringing in people for a long time to dig that tunnel from her kingdom to Narnia.
 * The Green Mist is not in any of the books, and thus is only canon to the movies' continuity.