Salo, or the 120 Days of Sodom (1976)
This is not technically a horror film. It is an Italian art film directed by auteur Pier Paolo Pasolini, who was murdered shortly after completing the film. Based loosely on the book by the Marquis de Sade, this film updates the story to fascist, wartime Italy. Eighteen teenagers are kidnapped by four rich industrialists and raped, tortured, and murdered. It is absolutely relentless in its depravity, and has influenced generations of horror films. Sometimes it takes a master to make the most violent film ever made. Which I would argue this film is.