Homeward Bound — Bruce Coville

When his father dies, Jamie is sent to live with his uncle, a dark, forbidding man who has possession of an alicorn - a unicorn's horn. Jamie becomes obsessed with the horn, and discovers that his uncle has another secret, a much darker one. Solving the riddle of his own life is the only way that Jamie can, finally, truly go home.

Originally published in THE UNICORN TREASURY.