III (2015) on Netflix

Plot summary

An unidentified disease has taken many lives in a small European town, and it has now reached Mirra, the younger of two sisters. Their mother is already dead, and doctors offer nothing that helps. Ayia, the elder, turns to Father Herman, the parish priest and a friend of the family, whose house holds books that sit far outside conventional religion. From them she learns that her only chance to save Mirra lies in entering her sister's sick subconscious and finding what truly caused the illness. The ritual will take her into the depths of that mind and the demons waiting there, and she must hold her nerve while she does it.