Town with a Dark Secret

Everybody in a small town used to have a normal life until something bad happened in their hometown long time ago. So years have passed, everybody from the town decide to keep it a secret.

A terrible secret that nobody outside the town must know. The visiting protagonist slowly begins to suspect that something is wrong and they'll try to stop the main antagonist before it's too late!

Examples
Haven, which is near Derry, in The Tommyknockers. It starts off as a normal town, but becomes this trope when the alien spaceship is first unearthed and slowly makes everyone crazy and obsessed with it. Ludlow has the secret of the real Pet Sematary, the Wendigo, and what lies beyond the deadfall...
 * In Toy Story 3, Woody and the gang wind up in a day care that looks like paradise. The toys (led by a fluffy stuffed animal named Lotso Huggin Bear) are all friendly, there are always lots of kids to play with them so that none of the toys ever get outgrown, and there's a repair ward that keeps the playthings in tiptop shape. However, their dark secret is that, in order to stay in the older kids' playroom, the ruling toys regularly sacrifice new toys to the toddler's playroom, where too-young children bash and beat toys until they are destroyed and thrown out.
 * In Children of the Corn 666: Isaac's Return, the citizens of former Ghost Town Gatlin act normally, but gradually drop it until it's time for the final prophecy.
 * Implied in the Friday the 13th (2009) remake, where at least one resident was shown to be aware of Jason living and killing in the old campgrounds. This is markedly different from the originals, where the residents of Crystal Lake didn't seem to know exactly what was going on at the camp, but were pretty clear in their warnings to outsiders.
 * In Freddy vs. Jason, many adults in Springwood know of Freddy's dream-killings, but have conspired to conceal this from the town's youth to starve him of the fear that gives him power over dreams. Jason comes to Elm Street at Freddy's instigation, so his killings will revive old stories about the Springwood Slasher and restore Freddy's powers.
 * In John Landis's An American Werewolf in London, the small English town of East Proctor's Dark Secret is, unsurprisingly, a werewolf.
 * The secluded suburb where Get Out takes place, where a number of young African American people have gone missing. The locals are members of the Coaguala, with the Armitage family as their leaders. The Armitage's run a secret business where they will kidnap a young African American person, scoop out most of their brain, and implant the brain of an old white person.
 * Author Stephen King calls this "The Peculiar Little Town" and has confessed that he has a weakness for writing stories of this type (among them Children of the Corn, Rainy Season and You Know They Got A Hell Of A Band).
 * His best known peculiar little towns are Derry (IT) and Castle Rock (good number of stories), both in Maine, which tend to redline the weird-shit-o-meter on a regular basis. In the end, Derry is destroyed, which is probably for the best. Castle Rock is destroyed by a visiting evil that took advantage of the secrets and flaws of many of the townspeople so that it could take their souls.
 * Jerusalem's Lot from 'Salem's Lot. It's had a family of gangsters that worshiped demons and consorted with vampires. One of these vampires comes to town and then it really has a dark secret. By the end, the whole town is undead except for the protagonists, who burn the town down and leave. A couple of later short stories by King reveal that burning the Lot down only temporarily halted the vampires. And it's hinted throughout both the novel and the prequel story that we still don't know the worst of its secrets.
 * Willow's inhabitants in Rainy Season try to warn people about their annual "bad weather problem," because warning the newcomers is part of the tradition. As is the inevitable death of the newcomers — always a young man and woman.
 * Little Tall Island has a couple. In Dolores Claiborne, it's the decades-long mystery following the death of Dolores' husband. In Storm of the Century it's the fact that the entire town surrendered one of their children (the police chief's son, no less) to a visiting demon to save themselves.