'Longlegs' haunting ending, explained

How are you feeling after seeing Longlegs? Unsettled? Spooked? Like you've got the teeth of the hydra upon you?SEE ALSO:'Longlegs' review: Nicolas Cage may be brilliant, but this horror-thriller is bizarre in the wrong way
That's more than understandable. The latest film from writer/director Osgood Perkins is packed with eerie atmosphere, Silence of the Lambs–style serial killer investigations, Satan worship, and a surprising number of references to the glam rock band T. Rex. All this lead