John Carpenter’s In the Mouth of Madness owes a hell of a lot to H.P. Lovecraft, though it doesn’t explicitly reference any single work of his. (That said, the title certainly brings to mind Lovecraft’s At the Mountains of Madness, which was going to be adapted into a feature film by director Guillermo del Toro.)
In the movie, the popular horror novelist Sutter Cane - sort of an amalgamation of H.P. Lovecraft and Stephen King - has gone missing, and P.I. John Trent, played by Sam Neill, has been hired to find him (or, at the very least, find his most recent manuscript so his employers can publish it). The search brings Trent to Hobb’s End, a fictional setting used in many of Cane’s works, where he encounters people, places, and real-life nightmares lifted directly from Cane’s books. Eventually, it becomes apparent that John Trent isn’t merely searching for the manuscript known as In the Mouth of Madness - he’s also living it.
In the Mouth of Madness is one of John Carpenter’s lesser-known horror movies after Halloween, The Thing, Christine, and The Fog, but it’s also one of his best - which is strange, considering it was made the same year as one of his worst, the dreadfully boring remake of The Village of the Damned starring Superman (Christopher Reeve).
In Mouth of Madness, from the first reel to the last, there’s this constant sense of something being wrong beneath the surface of what’s going on. As viewers, we only see glimpses of that wrongness, that underlying abomination, but that’s what makes this movie so damn good. Whereas famed filmmaker Stuart Gordon made some fantastic movies based on Lovecraft’s library -films you'll find on this list - those movies weren’t exactly accurate adaptations. After all, in addition to being incredible horror movies, they were also hilarious half-comedies.
My point is, despite not being a direct adaptation of H.P. Lovecraft’s work, John Carpenter’s In the Mouth of Madness is the best, most accurate cinematic representation of it as a whole. Time will tell if another filmmaker can usurp John Carpenter - maybe it'll be del Toro's At the Mountains of Madness, if it ever gets off the ground again - but for now, Carpenter reigns supreme.