The Lost Colony (Artemis Fowl, Book 5)

Until recently, Artemis Fowl was the only human to have discovered that magical beings do indeed exist. But now a second juvenile genius wants to capture a demon for scientific study. Only an ancient time spell separates the demons from humankind--and Artemis must prevent it from unraveling. If he fails, the bloodthirsty tribe will relaunch their quest to wipe humans from the planet.