Dark Cabal
This is the name of a cult that was founded in the 1930s and 1940s. It began in England and was headed by Sir Anthony Baskerville, who was to later become a follower of Baron Mordo, and the German Viscount Heinrich Krowler, grandfather and early instructor of Baron Mordo. Not only did they attempt to pave the way for the invasion and subjugation of the Earth dimension by the dread Dormammu, but they did so by creating an atmosphere of chaos by backing Adolph Hitler's government as well. They drew most of their mystical energies from the hate and chaos generated by the bombings of London.
Most of the original Members of the Dark Cabal were destroyed by the bombing itself, but some probably escaped and later founded new cults dedicated to Dormammu. It would certainly explain the presence of many that have cropped up later on.