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.