Dave Duncan 
Present Tense 

As WWI rages on, two immortal enemies engage in a war between alternate worlds in book two of this epic fantasy series. In the midst of the horror of the First World War, a stranger falls from nowhere into the mud and death of Flanders battlefield—bruised, babbling, and stark naked . . . with a remarkable story to tell. The Great Game—the timeless diversion of human gods, a ruthless contest of treachery, magic, betrayal, and manipulation, created to relieve the tedium of immortality—goes on.