The Secret Doctrine is an extensive tome filled with prophecies that would not reach fruition for many years. It describes the Brotherhood of Purity's objectives, as they are contained in the prophecies within the sacred text.
It was written in the third century AD by a group of Docetists, who attempted to summarise the order's history and doctrine and set in print the genealogy, philosophy, and icons. Heinrich Himmler, in charge of the Nazi Occult Bureau, somehow obtained a manuscript of the Secret Doctrine and showed it to Adolf Hitler. There are several reputedly authentic excerpts but there is no definitely original copy of the text.
The Secret Doctrine describes an entity called the Incubus, which the Brotherhood would summon when all its prophecies had come to pass. The Incubus would then create the Moon Child which would be the destroyer of the impure and the guardian of the pure and faithful.