Aleister Crowley was a British writer, poet, and occultist who was raised within a strict fundamentalist Christian sect named The Plymouth Brethren. He became interested in the occult while an undergraduate at Trinity College, Cambridge. In 1896, during a trip to Stockholm, he experienced his first mystical experience. In 1898 he joined the order of the Golden Dawn, shortly before it collapsed, and shortly before he left to explore the orient. In 1903 he married Rose Kelly who encouraged his exploration of the occult and his writing of the Book of the Law. They divorced six years later.

Crowley settled for a number of years in Sicily with a group of disciples at the Abbey of Thelema. Rumours of drugs, orgies, and magical ceremonies led to his expulsion from Italy. He spent much of the rest of his life developing a "Thelemic philosophy". In 1906 he founded a new order A.A. (Astron Argon, Astrum Argentium or Silver Star) to take over where the Golden Dawn had left off. In 1910, he joined the Ordo Templi Orientis (O.T.O.), a group of high-ranking Freemasons in Germany. He took over as head of the order in 1921. Both the A.A. and the O.T.O. continue to teach the Law of Thelema.

Aleister Crowley is arguably the most famous occult character of modern times.