Dr. Tarr's Torture Dungeon (1973)

A mysterious man is sent deep into the forest to investigate the bizarre behavior of the notorious Dr. Tarr. What he stumbles upon is the doctor's torture dungeon, a hellish asylum completely cut off from civilization and presided over by the ultimate madman. Innocent people have been savagely chained, tortured, and stuck in glass cages, then forced to take part in gruesome games of ritual slaughter.