The Secret Country Trilogy by Pamela Dean

Ted, Laura, Patrick, Ruth and Ellen are still trapped within the Secret Country - a land that they invented as a game, now somehow a reality. They must play their parts, fumbling as they go, even as life veers from their original script. When the King is poisoned, Ted - "Prince Edward" - must take the throne, although he has no idea how to rule a country, battle magic, or inspire followers. Soon enough he will have to do all three, because the Secret Country is on the verge of war with the treacherous Dragon King - a war that will send one of the five into the Land of the Dead...

There are three things that have the power to destroy the secret Country: the Border Magic, the Crystal of Earth, and the whim of the dragon. Ted, Laura, Ruth, Patrick and Ellen have faced the first and the second; now, summoned back to the Secret Country, they are up against the third. The country's most trusted counselors now konw that the five are imposters, somehow thrust into the roles of royalty. But no one knows who has been playing with their destinies, and that of the Secret Country - so they must find and speak with Chryse, the unicorn, and Belaparthalion, the dragon, in order to bring the truth to light once and for all. As with everything in the Country, this is both more complex and dangerous than is seems...