While The Conjuring movie series is famous for its real-life cases, the demon group the Disciples Of Ram featured in The Conjuring: The Devil Made Me Do It are not. The group is initially mentioned in the first Annabelle movie after a member attacks the central couple in the beginning of the film. Subsequently, a member of the Disciples Of Ram is the villain in The Conjuring 3, establishing the group's importance across the entire fictional universe.
The Conjuring 3 follows the case of the real-life Arne Johnson. Ed and Lorraine Warren are called in when his girlfriend's little brother is possessed. Arne invites the demon into him and subsequently commits a murder under its influence. The Warrens work to find tangible proof for Arne's case that he was not in a sound state of mind, and therefore not responsible for his actions. They find it in the form of a witch's totem.
The Warrens trace the totem back to Father Kastner, who researched the occult for years in order to better understand it. They make the shocking discovery that not only did the priest have a daughter, but she was a member of the Disciples Of Ram responsible for planting the totem. Although this mysterious cult is fictional, it does have some true story inspiration: namely, The Conjuring 3 uses the real-life fear of Satanism and the Occult that rose in the '70s and '80s, thanks to high-profile cases like the Manson Family murders and the Son of Sam killer. The Disciples of Ram cult reflects the public's fear (and misconceptions of) Satanism during that time period.
The origins of the Disciples Of Ram itself are never revealed in any movie across the Conjuring universe. They worship demons and seek to bring more of them into the world. Instead, the franchise peppers in information about specific members over the years. The earliest of whom is Janice Higgins, who eventually became possessed by the demon inside the Annabelle doll. Janice eventually took on the moniker of Annabelle and joined a mysterious group as a teen. That group was eventually revealed to be the Disciples of Ram. The cult comes back into play in The Conjuring 3 in the movie's climax. Ed and Lorraine reach out to Father Kastner, a priest who spent years immersed in the occult. His daughter rebelled against him and ended joining the Disciples of Ram, much to his dismay.
The repeated mentions of the Disciples of Ram demonstrate that The Conjuring universe intends to continue expanding beyond its existing franchises. The expansive universe was built by taking minor details and characters from The Conjuring movies and creating independent stories about them, such as the Annabelle doll and evil nun Valak from The Conjuring 2. By mentioning the Disciples of Ram again, The Conjuring: The Devil Made Me Do It has laid down the groundwork for the cult becoming an important element in the larger universe, perhaps even creating a movie around it in the near future. Detail-oriented moves like this are what have cemented The Conjuring series as a staple in the horror genre.