The Paradise War — Stephen R. Lawhead

From the shelter of academia, two students have come to the misty moors of Scotland to investigate the reported sightings of a mythical beast. Here where legends live, the inquisitive Simon Rawnson is seduced by the lure of the Otherworld -- and crosses a hidden border to serve as warrior for Nudd, lord of the eternal damned.

Now a breach has opened between the worlds. Wolves prowl the streets of Oxford. A mysterious Green Man haunts the Highlands. And an impending catastrophe that threatens to devastate two universes draws the skeptic, Lewis Gillies, into a realm of terrible magic--to save a friend...and a future.