What is the big twist near the end of The Witch?

The big twist near the end of The Witch comes when Thomasin—after witnessing the deaths and betrayals of nearly everyone in her family—makes a definitive choice: she accepts Black Phillip’s offer to “live deliciously,” signs his book (implying a pact with the Devil), and joins a coven of witches in the forest. She abandons her pious Puritan life and all the horrors she has suffered, embracing something darker but also liberating. In the final moments, Thomasin and the witches levitate; she herself ascends, surrounded by chanting and dancing, shedding her human bindings and stepping into a radically new identity.