The group became active when Lilith gathered people who had lost purpose, mostly women who were or could get pregnant. Lilith's 'help' made these people attached and devoted to her.
Lilith told her new followers to create a child for her that would be like her 'son', Sebastian Morgenstern—a Shadowhunter with demon blood. For this, the cultists performed experiments, injecting pregnant women—most or all also members of the cult—with Shadowhunter and demon blood. None of the children survived, however; they were born with deformities—clawed hands, specifically—and died shortly after. The men (members of the cult and the pregnant women's husbands) and women who were also injected with Lilith's blood became stronger and faster but also eventually became fatally ill.
The group was eventually discovered by Clary Fairchild and her Nephilim friends in October 2007. On the rooftop of the church, they confronted Lilith, and Simon Lewis was able to send her to the Void. Here, Sebastian was also resurrected—by the ritual Lilith had put in place before she was momentarily banished—and left with Jace Herondale.