Best-selling author Rick Riordan presents ten new stories—including one of his own—about beloved heroes that sprang from the imaginations of some of the best middle grade authors working today. "I hope you enjoy your trip through the multiverse mansion as much as I did. The real danger is that once you start exploring all the wonders herein, you may want to stay forever."—Rick Riordan A cave monster . . . an abandoned demon . . . a ghost who...