What happens to Matilda?
It wasn't just her removal from Penllynith that disrupted the ceremony when Matilda/Carys was a child; the cult also realized that to be a proper archangel vessel, the subject must come "with pure heart," meaning she must willingly enter the ceremony. The cult tricks Matilda into coming back to Dean House to save her little brother, David, whom they've taken. Because she came with pure intent, she's ripe for possession.
Sylvia shows Matilda into a crook in the Dean House basement, and the archangel does indeed take over Matilda's body. Her bones snap, her eyes glow black, and she hovers in the air beneath a full moon. The cult carries her body back into their circle, and Sylvia seems pleased that she pulled it all off, though she hints that Verity and Nick are becoming problems, and that she plans get rid of them.
But then Matilda's eyes snap open and we cut to two separate scenes: Verity and Stephen are tending to David when a door swings open and they look on in terror; Nick puts the kettle on in the kitchen and Matilda approaches him. Before we can see what happens to him, PC Graves comes to the house and finds Matilda and David -- and no one else. At the hospital, Matilda confesses that she isn't sure what happened. She blacked out at the mansion and doesn't remember a thing. She is reunited with her real mother, who is recovering from her suicide attempt, and her father, who was driven mad by the cult after Carys's disappearance.
Matilda excuses herself to the restroom, where she suddenly flashes back to images of herself viciously stabbing someone at Dean House. Her bones then crack and her eyes go black as we realize the archangel is still inside of her -- images of a tidied-up Dean House and four fresh graves in the surrounding woods reveal the extent of the archangel's violence. Stephen is one of the bodies buried there, as we see his watch sticking out of the ground, but it's unclear who Matilda spared, as there were five cult members. Nick is the most likely answer, as he wasn't directly involved in the prior events, but it's still anyone's guess.