Who's in the cult and what do they want?
The cult's members are Sylvia, a local antiques dealer; social worker Lloyd Satlow (Simon Kunz); his wife Verity (Pippa Haywood), a psychiatrist; and Stephen Kendrick (Brendan Coyle), a former Detective Inspector who worked on Carys' missing person case. They also temporarily lure in Nick Dean (James Frecheville), a distant relative of the Dean family who inherits Dean house after his uncle's death and has a sexual relationship with Matilda. Nick didn't know about the cult or its capabilities, but joins because he believes it will help him gain access to his inheritance. (If you're getting John Cassavetes in Rosemary's Baby vibes, you're not alone.)
Sylvia, whom Stephen later confirms is a powerful occultist, was drawn to Penllynith and Dean House because of its mystical properties. "This area has always been special," Sylvia tells Matilda of the land surrounding Dean House. "Milk tastes better, soil's richer, water's clearer. The druids certainly thought so. Maybe Theodorus Dee did, too." Silvia's ultimate goal is to summon an archangel so she can act as its second-hand, although it remains unclear what she hopes to achieve. It seems she's lured the others into the cult with promises of the divine; Stephen, for instance, claims that Sylvia will be able to save his sick son.