It is told that, one night, a Jewish merchant from Prague walked close to a ruin on his way home. A black dog ran up to him and circled around him, barking fiercely. The man was certain that the ferocious beast would tear him to bits with its vicious fangs. Somehow, the man managed to reach home alive and went straight to bed, traumatized and exhausted. Later that night a barking noise woke up his family. His wife noticed that her husband was barking in a dream and was bathed in sweat. The terror of his experience that evening had returned in his dream. Upon waking, he told his family that he had dreamed that he was riding a black dog in a column of dogs and riders, and he had barked like the others so that he would not be eaten alive. The dream recurred the next night and on subsequent nights, and the distressed man soon became too ill to fulfill his daily duties. His family realized that they had to find help.
They turned to Maharal, the great rabbi Judah ben Bezalel. The rabbi took the initial precaution of checking the man’s prayer shawl and phylacteries. He found them flawed and explained that the man was threatened by the black dog because the ritual objects no longer protected him. He ordered the man to replace the flawed objects with new kosher ones and immerse himself in a mikveh, the ritual bath, for ritual purification. He sent him home with an amulet for extra protection and specific instructions to sleep in the house of study, not at home, for the next seven nights.