While researching, I reread Carmilla and I remembered why I fell in love with the story to begin with. Le Fanu made powerful use of atmosphere. That is the core of good horror writing. By evoking rich images that the reader can relate to, you allow the reader to disappear into your world. Within that framework, slowly revealing the inevitable danger creates the suspense that makes horror effective.
I am hardly there, but I am learning. The more we read, the more we understand the way that writing works. There is no magical formula for good fiction, but there are tools that we can make use of to improve our writing. Take a look and see what I mean about building atmosphere. Carmilla at Project Gutenberg
Keep writing friends!