The Lost War (Unabridged‪)‬

It was supposed to be a weekend of costumed fun. Instead these medieval historical reenactors are flung into a wilderness by magic they don't understand. They must struggle to survive and deal with monsters who consider them prey...or worse.  

"Karl Gallagher's first production, the Torchship Trilogy, was good enough so that I read and reread it. He has now turned his hand from science fiction to fantasy." (Professor David D. Friedman, Professor, Santa Clara University, author of The Machinery of Freedom and Salamander; also known as Duke Cariadoc of the Bow, KSCA, OL, OP, founder of the Pennsic War)