The islands of Britain have been a crossroads of gods, heroes, and kings-those of flesh as well as those of myth-for thousands of years. Successive waves of invasion brought distinctive legends, rites, and beliefs. The ancient Celts displaced earlier indigenous peoples, only to find themselves displaced in turn by the Romans, who then abandoned the islands to Germanic tribes, a people themselves nearly overcome in time by an influx of Scandinavians. With each wave of invaders came a battle for the mythic mind of the Isles as the newcomer's belief system met with the existing systems of gods,...