What if HP. Lovecraft and John Carpenter made a film together? Well, that'd be kind of weird because HP Lovecraft died before John Carpenter was even born so there'd some pretty spooky shit going on for that to happen, but forget all that. "In the Mouth of Madness" is essentially that film.
When horror author Sutter Kane (your HP Lovecraft stand-in) disappears, private investigator John Trent (played by Sam Neill) is hired by his publisher to track him down and collect his overdue draft. Trent soon tracks Kane down to the small New Hampshire town of Hobb's End, which is troubling as the town exists only in Kane's fiction. After discovering that Hobb's End is a real place, Trent soon learns that things in town aren't what they seem and that Kane's work may not be as fictional as everyone thinks.
The final film in Carpenter's "Apocalypse Trilogy" (the other two being "The Thing" and "Prince of Darkness"), "In the Mouth of Madness" is pure Lovecraftian terror as translated by John Carpenter. Where other Lovecraft adaptations fail because of an unwillingness to stray from the source material in the slightest, this film manages to simultaneously nail all those classic Lovecraftian tropes will still feeling like a Carpenter film.
One of Carpenter's last truly great films, everyone should check this one out at least once. If nothing else, I can guarantee you it's better than "Ghosts of Mars".