The Green Lady and the King of Shadows by Moyra Caldecott

When Lukas stumbles into a hidden tunnel under Glastonbury Tor, he is unprepared for the events that follow. He meets the hermit Collen and assists in the struggle against the mighty Gwyn ap Nudd, a confrontation involving the Earth Goddess and, ultimately, the highest powers in the universe.

Drawing on folklore, myth and legend, Moyra Caldecott weaves a thrilling fantasy of the struggle between good and evil in Dark Ages Britain.