Straight from the map editor of Prism comes The Quest of Sir Markus (details), an extra-large single-player map for H3: The Restoration of Erathia. "You are the Heir to a kingdom and must prove your worthiness with a royal and lengthy quest." The mapmaker recommends that you search each area extensively for any secret. The quest won't be easy, so you will need all stat improvements and creatures you can find.

A Heroes III map gets released more than seven years after the release of its game, while Heroes V is still in the new releases at your local store. I guess it shows the marvels that a good map editor can do to the lifespan of a game. Ubisoft, I hope you are taking notes!