Aleister Crowley (/ˈkroʊli/; born Edward Alexander Crowley; 12 October 1875 – 1 December 1947) was an English occultist, ceremonial magician, poet, painter, novelist, and mountaineer. He founded the religion and philosophy of Thelema, identifying himself as the prophet entrusted with guiding humanity into the Æon of Horus in the early 20th century.

Born to a wealthy Plymouth Brethren family in Royal Leamington Spa, Warwickshire, Crowley rejected this fundamentalist Christian faith to pursue an interest in Western esotericism. He was educated at the University of Cambridge, where he focused his attentions on mountaineering and poetry, resulting in several publications. Some biographers allege that here he was recruited into a British intelligence agency, further suggesting that he remained a spy throughout his life. In 1898 he joined the esoteric Hermetic Order of the Golden Dawn, where he was trained in ceremonial magic by Samuel Liddell MacGregor Mathers and Allan Bennett. Moving to Boleskine House by Loch Ness in Scotland, he went mountaineering in Mexico with Oscar Eckenstein, before studying Hindu and Buddhist practices in India. He married Rose Edith Kelly and in 1904 they honeymooned in Cairo, Egypt, where Crowley claimed to have been contacted by a supernatural entity named Aiwass, who provided him with The Book of the Law, a sacred text that served as the basis for Thelema. Announcing the start of the Æon of Horus, The Book declared that its followers should adhere to the code of "Do what thou wilt" and seek to align themselves with their Will through the practice of magick.

The D.Gray-man anime and manga series features a cast of characters created by Katsura Hoshino. The series is set at the end of a fictional 19th century during which the Black Order searches for Exorcists, the "Apostles of God" chosen to wield a divine substance called Innocence, as they are the only ones who can destroy Akuma, weapons created by the Millennium Earl from souls of the deceased when humans curse God for the loss of loved ones, and stop the Earl's plan to destroy humanity.

The main character of the series is Allen Walker, a young Exorcist who trained under Exorcist General Cross Marian and is sent to officially join the Black Order. Early in the series, he is sent on missions with fellow Exorcists, on which he meets Yu Kanda, a cold and mysterious swordsman looking for a certain person from his past; the friendly Lenalee Lee, who has been an Exorcist her entire life; and Lavi, the cheerful and energetic future Bookman, chronicler of the secret history of the world. They travel the globe, retrieving the pieces of Innocence scattered during the Great Flood and recruiting new Exorcists while fighting the Earl and his Akuma. However, the Black Order soon learns that the Earl has help in the form of the Clan of Noah, a group of thirteen super-humans who can destroy Innocence and claim to be the true "Apostles of God." Both groups quickly begin searching for the Great Heart, the most powerful Innocence that can strengthen or destroy all the others, to bring an end to the war.