A major driver of the film's "scariest movie" reputation is its connection to the Vallecas Case , the first instance in Spanish history where a police officer officially recorded unexplained paranormal activity in a report.

Director Paco Plaza clarified that while the police report was the starting point, the movie is a fictionalized "vision" rather than a documentary. Movie Highlights & Themes

The film's success eventually led to a prequel, ( Hermana Muerte ), which was released on Netflix in 2023.