Thomas Carnacki was an occult detective who became a member of the 20th Century League of Extraordinary Gentlemen.
Prior to the League
Thomas Carnacki earns his reputation as being a prominent occult detective, though a pale shadow of the greatness of Sherlock Holmes.
Carnacki was recruited by the League in 1909. A year later in 1910, Carnacki had visions of bloodshed on the waterfront and of an apocalypse caused by Oliver Haddo bringing about the birth of a Moonchild. Carnacki informed the League of his vision. The League tries to gather information from a mirror as well as files on Haddo. However, when the League directly confronts Haddo in his own residence he is innocent for the time being. In fact, Carnacki's words inadvertently hint to Haddo to search for the person in his vision in order to give birth to the Moonchild.
Carnacki and the League saw Jack MacHeath about to be hung, but a last minute pardon saves MacHeath. Carnacki and the League are then informed of an attack on the docks. Mina assumes that the attack was the massacre that Carnacki foresaw, but Carnacki said it was not, and that it could perhaps be even decades before it is ever seen. Instead the massacre was committed by Captain Nemo's daughter, Janni.
Three years later, Carnacki was later involved in the battle between the League's French counterpart, Les Hommes Mystérieux, in which Carnacki had also foreseen including visions of World War I. Carnacki's visions further helped in mistakenly antagonizing France for provoking World War I due to machinations of Die Zwielichthelden. Carnacki, however, did not joined his colleagues in France due to being feverishly ill. The battle had no clear victor. By 1938, Carnacki retired from the League due to old age.