Dark Island

Dark Island located in the Eastern Sea was the island where dreams come true. It was basically a mass of solid darkness, with no obvious shore (except, perhaps, in dreams). Caspian X and his crew attempted to pass through this island on their voyage in the Dawn Treader. Lord Rhoop was trapped on this island for a time, and when rescued said that it makes nightmares come true and requested never to be asked what he saw during his stay. Then the Dawn treader sailed away with the island fading off into the distance.

Differences between versions
In the British version and now all recent publications have the Lord Rhoop asking Caspian never to take him back to that island, and when the Dawn Treader leaves it they look back to find the island gone, destoryed by Aslan.