III (2015)

A small European town, where sisters Ayia and Mirra live, gets struck down by an unknown disease which takes many lives. Following their mother's death, the younger sister falls ill. Having realized that conventional medicine is useless in the face of the sister's disease, Ayia seeks help from Father Herman, a parish priest and a close family friend. In his house she finds books that are very far from the conventional religion. She gets to know that only penetration into Mirra's sick subconscious mind and discovery of the true cause of her disease will give her a chance to save her sister. Ayia is ready to go through this terrifying ritual, dive into the depths of the subconscious mind, and face the demons residing there. But will she cope with her own fear when she discovers the mysteries of her sister's past? Since the closer to the bottom of the ocean, the darker it gets.

Source: TMDb