The Heretics (2017) online

A notorious cult kidnaps a young girl, and sacrifice themselves by the light of the locust moon. The next morning the girl awakes, caked in dried blood and surrounded by corpses...but safe - or so she thinks. Years later, the locust moon is about to rise again and the girl is captured once more by a surviving member of the cult. She is taken to a remote cabin where she learns that a demon has been growing inside of her all these years, and before the dawn it will rise.