Hellraiser (1987)
Film
Hellraiser is a horror film primarily centering around the antagonists Julia and Frank Cotton, as well as Julia's stepdaughter, Kirsty. It is more famous for introducing the iconic horror villain Pinhead and his gang of Cenobites.
The film opens with Frank finding a strange puzzle box, the Lament Configuration*, which kills him upon completion. After he is partially brought back to life, he seeks help from Julia to fully resurrect him.
While it's relatively well known, it still seems to be overshadowed by the many other horror films of its decade, which is why I consider it a cult classic. It's unconventional in its scares and focuses heavily on philosophy as the main source of its horror.
There are numerous sequels, which I still have yet to watch, plus a few comic books. The author of the original story, Clive Barker, has also gone on to write a few sequels and side-stories. Cradle of Filth released the album Midian in 2000 which is partially inspired by his works and features the actor of Pinhead (Doug Bradley) as a guest narrator.
The philosophical depth to the film, as well as the amazing use of visuals, is what makes it such a terrifying masterpiece. It is Lovecraftian. Feeding upon existential dread to fuel its fire, Hellraiser is absolutely unparalleled and will not be easily forgotten.
* - It is not referred to as the Lament Configuration in the film, although this is the name it has since been given through the sequels and comic books.