As the logline from Warner Bros. states, the sister's "reunion is cut short by the rise of flesh-possessing demons, thrusting them into a primal battle for survival as they face the most nightmarish version of family imaginable."

Source: https://collider.com/evil-dead-rise-streaming-date-max/

This happens after Danny (Morgan Davies) discovers a book in the bowels of the building that summons demonic flesh-possessing demons.

Source: https://screenrant.com/evil-dead-movie-tv-series-timeline-explained/