The Secret Country

For the past nine years, cousins Patrick, Ruth, Ellen, Ted, and Laura have played at "The Secret"-a game full of witches, unicorns, a magic ring and court intrigue. In The Secret, they can imagine anything into reality, and shape destiny. Then the unbelievable happens: by trick or by chance, they find themselves in the Secret Country, their made-up identities now real. They have arrived at the start of their game, with the Country on the edge of war. What was once exciting and wonderful now looms threateningly before them, and no one is sure how to stop it...or if they will ever get back home.

Synopsis from Open Library.