The origins of this group have been lost in time. Some say that it existed in the days of Atlantis and ancient Mu. Some have hinted to a connection with K'n-yan. According to a Mi-Go posing as Henry Akeley, they are dedicated to the god Hastur, and they have a long-standing goal of eradicating the Mi-Go.
One modern, paramilitary branch of the Brotherhood is The Army of the Yellow Sign.