Oliver Haddo was a black magician. In 1908, Haddo barely survived a fire at his Staffordshire estate and later took the alias of Dr. Karswell Trelawney.
In 1910, Oliver Haddo came to the attentions of the League when their member, occult detective Thomas Carnacki had visions of Haddo convening to plot the creation of a Moonchild destined to bring forth the end of the world. Haddo encountered Orlando, Allan Quatermain and Carnacki when they stumbled into his circle of magicians. In response to Carnacki's accusations, Haddo pointed out to him and his colleagues that what Carnacki saw either is wrong, or has not happened yet. In fact, Carnacki inadvertently gave Haddo a crucial piece of information that they need to create the Moonchild.
Oliver Haddo later contributed his service to the British government during World War II. During the post-war era, Haddo wrote the On the Descent of the Gods based on his collective research on the history of the paranormal. Reportedly, Haddo died destitute in Hastings in 1947. However, Haddo's spirit lived on and possessed Kosmo Gallion, and from there created the Cult of Gallion to continue his plan for the Antichrist. In 1969, Haddo intends to transfer his spirit to the body of Terner, the leadsinger in the band Purple Orchestra. As Haddo prepares his ritual, he is confronted by Mina Murray on the astral plane, Haddo overpowers Mina, though he reveals that his possession of Terner will not effect his planned birth of the Antichrist. Unfortunately, the plan gone awry as Gallion was killed by Jack Carter for the murder of Basil Fotherington-Thomas. Haddo was then forced to enter the body of Tom Riddle.
As Tom Riddle, Haddo attended an "invisible college" devoted to teaching magic and subsequently becoming headmaster. Haddo soon founded the Antichrist, a student marked as a baby by Haddo with a distinctive scar who was subsequently manipulated into accepting his destiny through various adventures against an arch-nemesis portrayed by Haddo himself. The Antichrist came to learn the truth behind Haddo's manipulations and was driven mad, so he went into a rampage on the college and massacred its students and faculty. Haddo was decapitated by the Antichrist, who took his still-living head with him. Haddo's living head then spent the next decade in a cage in the Antichrist's house, while the Antichrist medicated himself with antipsychotic pills, and tried to put-off his eventual role in the Apocalypse.
The Antichrist was eventually confronted by Mina Murray and Orlando. God, in the form of a woman named Mary, then intervened by turning Haddo's Antichrist into a chalk drawing, preventing the Apocalypse and saving mankind. Haddo's head was taken away by God/Mary Poppins, and never seen again.
Oliver Haddo is from W. Somerset Maugham's novel The Magician (1907). Haddo was based on Aleister Crowley, whom Crowley later used “Oliver Haddo” as a pseudonym.