Cthulhu Mythos cults
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A number of fictional cults appear in the Cthulhu Mythos. Many of these cults serve the Outer God Nyarlathotep, the Crawling Chaos, a protean deity that appears in a myriad of guises. Other cults are dedicated to the cause of the Great Old Ones, a group of powerful alien beings currently imprisoned or otherwise resting in death-like sleep.
The Black Brotherhood is a group of international terrorists dedicated to hastening the return of the Great Old Ones. The Brotherhood is made up of people from all ethnicities and walks of life, and, believing that the Old Ones will awaken soon, conduct covert operations, such as attempted political assassinations. Few of its members are taken alive for interrogation, however, as most die shortly after carrying out their operations.
Brotherhood of the Beast
The Brotherhood of the Beast is a cult that worships Nyarlathotep in his aspect as The Beast. The Brotherhood is dedicated to fulfilling the prophecies of its founder, Nephren-Ka, an Egyptian high priest of Nyarlathotep in the 14th Dynasty.
Brotherhood of the Black Pharaoh
The Brotherhood of the Black Pharaoh serves Nyarlathotep in his aspect as the Black Pharaoh. Its leadership is primarily of Egyptian descent, though in modern times it has become more inclusive of other nationalities. The cult has connections to the Church of Starry Wisdom, the Cult of the Bloody Tongue, and the Brotherhood of the Beast. It includes a subgroup known as the Children of the Sphinx that specializes in embalming mummies with the heads of animals.
Brothers of the Yellow Sign
The Brothers of the Yellow Sign (or Cult of the Yellow Sign) worship Hastur in his aspect as the King in Yellow. The cult is led by a group of human immortals from the technologically and physically advanced realm of K'n-yan (the cult probably originated in this subterranean land, though others trace it to the serpent people of Valusia).
Chesuncook Witch Coven
The Chesuncook Witch Coven, or the Cult of the Skull, worships the Outer God Shub-Niggurath. They hold their ceremonies in the deep woods near the village of Chesuncook, Maine. Their rites supposedly include entering a subterranean cavern and descending six thousand steps to a pit filled with hungry shoggoths.
The Chorazos Cult is a sect dedicated to the Outer God Yog-Sothoth. The cult was founded in the mountains of Romania in the late 16th century, but later moved to England and then Scotland. The cult developed an unwholesome reputation and was eventually disbanded by locals. The cult returned in modern times, at least superficially based in the United States and attempting to awaken Yog-Sothoth. Its name suggests an allusion to Choronzon.
Church of Starry Wisdom
The Church of Starry Wisdom, or Starry Wisdom Cult, is a cult that worships Nyarlathotep in his aspect as the Haunter of the Dark, as described in the story of the same name. The cult was founded in Providence, Rhode Island circa 1844 by the archaeologist and occultist Professor Enoch Bowen. The cult used a sacred relic known as the Shining Trapezohedron to summon the Haunter of the Dark, who demanded outrageous sacrifices in return for limitless knowledge of the universe.
Although the cult was publicly denounced by the other local churches, it nonetheless grew to have a membership of around 200 members. Mysterious disappearances in the area brought the cult under public scrutiny. After run-ins with both local citizens and the municipal government, the church closed and 181 people left Providence for unknown reasons. However, there is veiled evidence that they were dealt a more summary kind of justice than banishment.
Though the Providence sect was officially disbanded in 1877, the Church of Starry Wisdom has appeared in other places including Yorkshire, England (1880 to 1890), Chicago, Illinois (where it was known as the Celestial Providence sect, but was later disbanded by the Great Chicago Fire of 1871), Arkham, Massachusetts (during the 1920s), and more recently in San Francisco (though the group was reputedly destroyed by arson). A few contemporary sects may still exist in Canada. As recently as the 1970s the cult was present on Bioko (Fernando Po) in Equatorial Guinea.
The Church of Starry Wisdom, also known as St. John's Church on Federal Hill in Providence, Rhode Island and was built and completed on March 17, 1873. The Builder of the Church is known as Patrick McEvoy who arrived in Providence from Ireland in the spring of 1843. Patrick McEvoy is a real-life character that is not present in any Lovecraft stories. Upon completetion of the Church he suffered a fall off one of the risers and died two weeks later of a staph infection to the knee. His wife Catherine Riley McEvoy died suddenely six weeks later of unknown causes. His eight children were left orphaned at the their home on Chalkstone Hill. (Berlin Street) The Children were neglected by The Catholic Church and never paid for their father's efforts. The third daughter Ellen McEvoy married Michael Lynch and took over ownership of the house and the hardware store across the street from the church on Federal Hill. The Diocese of Providence neglected the church and closed it down due to the Pagan/Gaelic style of the church and saw it unfit for the lofty and extravagant Italians that were quickly taking over the neighborhood. 
Cult of the Bloody Tongue
Esoteric Order of Dagon
The Esoteric Order of Dagon was the primary religion of the town of Innsmouth, Massachusetts in the early 20th century. Its members were dedicated to the worship of Dagon, Mother Hydra, and Cthulhu.
- Harms, "Black Brotherhood", p. 23.
- Harms, "Brotherhood of the Beast", pp. 38–9.
- Harms says that Keith Herber originally cited "Nophru-Ka" — probably a misreading of "Nephren-Ka" — as the leader of the group in his role-playing game supplement The Fungi from Yuggoth (which introduced the Brotherhood). In later works, Herber cites "Nephren-Ka" as the former leader of the group. (Harms, "Nophru-Ka", pp. 214–5.)
- Harms, "Brotherhood of the Black Pharaoh", p. 39.
- Harms, "Brothers of the Yellow Sign", pp. 39–40.
- Harms, "Cult of the Skull", p. 68.
- Harms, "Chorazos Cult", p. 53.
- See the ARG "Eldritch Errors"
- Harms, "Starry Wisdom Cult", pp. 285–6; "Dexter, (Doctor) Ambrose", p. 86.
- Robert Anton Wilson and Robert Shea, The Illuminatus! Trilogy
- J. M Lynch. (1992) Builder's and Razor's. A Historical account of Masonary and Church building in Providence
- Harms, "Cult of the Bloody Tongue", p. 68.
- Harms, "Esoteric Order of Dagon", pp. 103–4.
- Harms, Daniel. The Encyclopedia Cthulhiana (2nd ed.), Chaosium, Inc., 1998. ISBN 1-56882-119-0.