Subordinate clauses are those that cannot exist on their own, when they are not part of the independent clause. They always sound and look incomplete, because they are - they need to be included into an independent clause in order to be grammatically correct.
In this sentence, the subordinate clause is who wrote Gothic and macabre short stories. The type of the sentence is an adjective clause, because it begins with a relative pronoun who and the entire clause modifies the noun author, which is why it has to be an adjective. Adverb clauses would modify verbs or other adjectives.