The Cult of the Darkhold is a fictional organization in the Marvel universe.
The Cult of the Darkhold (aka, the Chthonic Brotherhood and the Darkholders) dates back to the Thurian Age, tens of thousands of years ago. It started out as the followers of the dark sorcerer, Thulsa Doom. Whom after his death, used the Darkhold to create the first vampire (Varnae). He turned several of its members into vampires. The survivors were able to escape with the Darkhold before Atlantis sank.
It's members either worship Chthon (one of the Old Ones) and the other Old Ones, and/or they have joined the cult to achieve wealth or power. In the modern era they have been opposed by the Darkhold Redeemers, the Avengers, the Midnight Sons, the X-Men, Spider-Man, Thor, Wolverine, and Captain America. The powerful 1,400+ yr old sorceress Morgan le Fay is a member of the organization and the legendary 500+ yr old Romanian vampire, Dracula, is likewise an ally.