Established by the
Freemasons in England in 1888, the Order of the Golden Dawn was a
fraternal organization with heavy ties to the
occult.
Led by S. L. MacGregor Mathers (1854–1917), the order's rituals were based on writings by Fred Hockley and membership depended heavily on mystic "skills" and generally weird-ass spooky habits.
The charismatic Aleister Crowley joined in 1898 and was expelled two years after, at which time he founded the rival "cult" Argenteum Astrum.
The Order of the Golden Dawn heavily influenced Yeats and, to a lesser extent, Algernon Blackwood.