Trappings is another word for characteristics associated, in this case, with style of writing. Trappings are things added on to something to make it more effective and attractive. If for example you are the Mayor (Chief citizen) of a big town, the trappings are the gold chain you wear, the special hat and so on. Supernatural means mysterious - out of this world. The use in your sentence suggests that whenever the writer describes terror he always includes the mysterious elements associated with fear and horror.