Oliver Haddo's first attempts to create his antichrist began in 1908, on his property, where the magician spawned a series of homunculi in an attempt to build a vessel. The attempt failed and Haddo nearly died in a fire that same year, prompting him to go into hiding and adopt the name Karswell Trelawney.
In 1910, Oliver Haddo came to the attention 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.
It is assumed that over the next few decades, Haddo and his followers tried several times to find a perfect candidate to be the prophesied moonchild. Some examples might include the rituals performed by Cyril Gray with the young Lisa la Giuffria just before the first war, or a mercurian policeman Jack Flash.
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 lead singer 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 affect his planned birth of the Antichrist. Unfortunately, the plan went 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.
Within the bosom of wizard society, Haddo was finally able to complete his plan, and in the year 1980, he finally managed to create his Antichrist in the form of a young orphan with magical powers. The child, marked by Haddo himself, would grow up to attend an invisible college, where he would live several adventures, which served to prepare him for his eventual role in the apocalypse.
the concept of the "Moonchild" is lifted from Aleister Crowley's 1917 novel of that title, as well as the christian concept of Antichrist.