The devil goes by many names, and his tribe is legion. Throughout human history, we have been obsessed with the dark opposites of God and angels, light, and mercy. Whether it is our religious and sacred texts, folklore and myths of old, legends, fairy tales, novels, or the movies and television shows of today, the dark entities enthrall us, terrify us, and remind us of the dualities of life. But where did they originate? Are they real? Does every religion or region of the world include them?

Exploring over two dozen religious traditions, myths, folkloric and spiritual traditions, the...