As a massive fan of Lovecraft and Call of Cthulhu, I have often found myself in the middle of a Pathfinder session shifting the overall mood to something more horror based than swords and sorcery. Thankfully I have some players who love adding scares and really dark tones to the adventure.
A great example has been when the group I was DMing entered a village called Ambertown and found that all of the inhabitants were almost sickeningly nice. Uncomfortable with such niceties, the group went out to cause a little trouble. However it seemed like it was impossible for them to get into trouble or be attacked by anyone with all of their attempts thwarted easily. The barbarian tried to throw a bottle of scotch at the bartender but he simply caught it on an attack roll of 23 and placed it back on the shelf before returning the barbarian's silver that he had paid it for. Eventually the group was plotting to burn down the village tavern as their paranoia increased after one of the bar boys who was nothing but smiles whispered to them to leave as quickly as they can. Almost immediately after this, the bar boy was led away by guards and they were too freaked out to even try to fight the guards. The adventure eventually developed into a scare riddled campaign as they tried to find a way out of Ambertown as leaving the village only led them back to it an hour later no matter what direction they left in. They really worked their detective skills and found themselves infiltrating the mayor's house and trekking through the home of the local undertaker. They eventually managed to escape through a powerful Gate spell that the wizard had to make harnessing powerful artifacts found around the village after the only combat of the entire adventure against a few of the local priests and villagers had killed three of the players. Since then, the village of Ambertown always makes the group skeevy and they have actively avoided any hints of the village returning or quickly leave when they see some of the smiles that echo the smiles of the citizens of Ambertown.
Have any of you ever found yourself shifting your DnD game to something more horror based midway through?
I do enjoy tossing in a bit of eldritch weirdness now and then. Whippoorwills in the distance, suddenly swarming to capture the soul of a dying "bad guy". Tree's swaying when there's no breeze. Stuff like that. I like leaving such weird moments unexplained and letting player's imaginations run wild.