Dagon

A young stock market tycoon, Paul, has been plagued by strange dreams of a green-eyed mermaid. His girlfriend, Barbara, feels it is most likely an indication of excessive stress and tries to convince Paul to go on vacation with her and their older friends, Howard and Viki, to the sunny shores of Italy. Paul, however, is reluctant to go citing work, though his girlfriend won't take no for an answer and the four of them set sail in a yacht for Italy.

Everything goes fine at first. However, a sudden storm near a small fishing village named Imboca dashes the yacht against the rocks. A large rock has pierced the cabin below deck and Viki's leg is trapped between the rocks and the yacht. Luckily, the yacht is otherwise stable and in no risk of sinking. Howard stays behind to care for his injured wife while Paul and Barbara go ashore to look for help....

Dagon is a 2001 horror movie loosely based on the Cthulhu Mythos short story The Shadow Over Innsmouth by H.P. Lovecraft. The film moves the action from New England to the coast of Spain, in the town of "Imboca" (which means the same as "Innsmouth"). Dagon is a highly atmospheric horror film steeped in mist and eerie themes. Despite being a modern update, many of the themes remain the same. Even if much overlooked, Dagon is worth a watch for many horror and Lovecraft fans.

Tropes used in Dagon include:

Apocalyptic Log: The rise of Dagon.
Barrier-Busting Blow
Bloodier and Gorier: Than Lovecraft, who was a advocate of Nothing Is Scarier.
B-Movie
Bilingual Bonus: English and Spanish are present here.
Bittersweet Ending: Paul may ultimately find happiness in capitulating to his inhuman nature and committing lots of incest with his dream girl, but the evil heathen fish monsters win, they've killed all his human friends, and he failed to save Barbara from being raped, impregnated, and murdered by Dagon.
Body Horror: Those fish people not all of them were born that way.
Campbell Country: The Spanish version.
Child by Rape: You're going to submit to Dagon whether you want to or not.
Cosmic Horror Story: By association with the Cthulhu Mythos.
Cruel and Unusual Death: Surprisingly frequent. A few couple include being skinned alive to make ceremonial masks and suicide by pocket knife Hara-kiri style.
Cthulhu Mythos: Although the deity is named Dagon after the character from the Lovecraft short story of the same named, it's implied in this movie that the deity is really Cthulhu -- as is strongly indicated by the cult constantly chanting "Iä! Iä! Cthulhu fhtagn!"
Defiant to the End Paul tries this three times. The first time, when the cult is flaying Ezequiel's face, they chant the usual "Ia Ia Cthulhu Fhatgin" mantra. Paul tries to put on a brave face for both of them by counter chanting with Psalm 23. Then when it's his turn, after begging the cult to simply let him, Ezequiel and Barbara go; he defiantly tells the priest "DO IT!". Later on, when Paul finds out that he's Uxia's sister and that they are both children of Dagon, he tries to burn himself alive so that he won't be Uxia's consort. Uxia stops him by pushing him in the water. Forcing Paul to accept his fate.
Eldritch Abomination: Dagon itself.
Extremely Dusty Home: As a citywide phenomenon.
Fan Service
Fish People: Some of them have cephalopod traits as well.
Flaying Alive.
Ghost City: Imboca At least at first....
Gorn
Hell Hotel: Barbara picked a great place to look for help.
- As Paul discovers, it comes complete with a Disgusting Public Toilet, smashed windows, and a bed in which someone--or something--has apparently died recently.
Horror Films
Humanoid Abomination: Paul.
Human Sacrifice: Viki and Barbara. Barbara gets more ceremony, though.
Kill It with Fire: the way roughly half the town meets its end.
Lovecraft on Film: An adaptation of both the short story Dagon and The Shadow Over Innsmouth.
Luke, I Am Your Father: said by one of the fish people to Paul
Malevolent Masked Men: wearing human faces no less.
The Mermaid Problem: Averted via tentacles with certain *ahem* functions.
Monster Progenitor: Dagon
Mood Whiplash: The slow pacing and dramatic mood of the film is interrupted by a flashback to a surprisingly dramatic depiction of the town's corruption by inhuman forces.
Nightmare Face
Ominous Fog
Only Sane Man: The old drunk Ezequiel; though he admits to being crazy in his own way as well.
Our Mermaids Are Different: Yes, very different. They are all the spawn or followers of an Eldritch Horror, they have no qualms about killing humans, and tend to have tentacles for just a start.
Religion of Evil: The cult of the fish god, Dagon.
Scenery Gorn: Also the city of Imboca with it's flooded houses, caving in roofs, and broken windows everywhere.
Shallow Love Interest: From a certain perspective, both of Paul's girlfriends. He's not fated to be with the one, and the other with whom he is fated to be is a girl he hardly even knows.
Shout-Out: That's a Miskatonic University sweatshirt Paul is wearing. Maybe he'd have fared better if he'd brought some of his old colleagues from there with him.
Shown Their Work: The movie is actually almost completely faithful to Lovecraft's themes and stories. The director and producer also did extensive research on comparative religion, which shows in the design work regarding the various implements and decorative items the cult uses.
Sinister Minister: The town's priest.
Sinister Scraping Sound: More like thump, shuffle and scrape.
Tomato in the Mirror: Paul is half Deep One.
Town with a Dark Secret
Unscaled Merfolk: They're more cephalopod than traditional merfolk or Fish People.
Yandere: Uxia. When the cult has Barbara, Paul and Ezequiel recaptured, she saves Paul from them moments before he was about to end up like Ezequiel. She claims that Barbara is destined to be Dagon's newest consort, despite Paul's pleas to let Barbara go. Even offering to accept being her lover. Yet, she not only says no, but later on during the actual sacrifice scene, Uxia takes a sadistic glee in carving the symbols on Barbara's naked body, before lowering her into Dagon's pit. One could just guess from the look on her face while doing so, that she was bullshitting Paul and doing everything she can to separate the doomed couple so she could have Paul all to herself.