Complex sentence: The Lovecraft oeuvre is sometimes divided into three periods the first two influenced by Edgar Allan Poe and Lord Dunsany respectively, but ideas associated with the third "Cthulhu Mythos" period were introduced from the earliest stories, and the classification is not exclusive.
Split into simpler sentences: The Lovecraft oeuvre is sometimes divided into three periods the first two influenced by Edgar Allan Poe and Lord Dunsany respectively. Many of Lovecraft's ideas associated with the third "Cthulhu Mythos" period were introduced in the earliest stories, and the classification iis not exclusive.