well i donno when i was reading the shattering they talked about the eyes of the earthmother the sun and the moon an'she(sun) and Mu'sha(moon also supected to be elune) in game there were two nps who were talking about how the tauren get the power of the Sun/light (so to speak) from an'she it is possible that they harnest the power of the shadow from Mu'sha
well that my personal take on that untill blizz release something about shadow priest lore