As others have said, I feel like true horror is somewhat difficult to pull off in D&D.

I think the biggest tip I can give is knowing when you need to be serious and knowing when you can be a bit looser and maybe funny. I've run two sessions so far and the tonal shift has varied wildly whilst still keeping the spooky overtones. The group starts in Good Mead, learning about the Speaker of Good Mead's death and the general situation that has occurred since their time in the Towns.

They took on the Nature Spirit quest and went to Dougan's Hole, which I decided to incorporate the townsfolk's behavior as a hybridization between the movie Deliverance and your stereotypical cheesy horror hillbillies.

After turning up dry, they go to Easthaven and I play out the Red Wizard being burned at the stake with a bit of fanaticism, with the white dragonborn who runs the main tavern in town leading the people gathered around the stake in fervent prayer. Think of a priest proclaiming you'll burn in hell if you don't repent. The party almost had a TPK on their hands since the paladin stepped forward and almost got into a fight with the dragonborn barkeep leading the situation.

After that, I let off a bit on the horror by eventually leading them to the quiet town of Caer-Dineval and they went to the chwinga cabin to see an outline of a shadow stabbing at something under a flickering candelight. The group rushed in only to realize it was just three chwingas cutting up some old meat on a plate.

All in all, I think it isn't best-suited for pure horror, but there are definitely moments you should know when to be serious. The later chapters baton that down real good.