Showing The Thirteenth Unicorn by W. D. Newman

While spending the summer with their grandparents on a remote farm in South Carolina, Ben Alderman and his sister Casey uncover a hidden world of magic; a world their grandmother is secretly visiting. It is a world where elves and dwarves are locked in mortal combat against a witch who is trying to free the last surviving wizard from exile. The witch has been defeated once before but with the combined power of the wizard, no one will be able to stand against them. Much to Ben's dismay, he learns that he is the one foretold in the ancient elfin prophecies to bring about the downfall of the witch and save this exciting new world from destruction.