Previously published as four separate books with a somewhat different text.
"This book is also available in four library-bound editions: Skyfall, Dark tower rising, The lava crown, Fallen star"--Title page verso.
Summary
Meteors fall above the town of Zion Falls, and below the town an ancient evil awakens--caught in the middle are four friends, who must use their new powers to defeat the troll monsters and save the world.