Complex sentence: It is the third and last novel set in the Alastor Cluster, a vast section of stars and planets ruled by the mysterious Connatic, which may or may not be a part of Vance's Gaean Reach.
Split into simpler sentences: It is the third and last novel set in the Alastor Cluster, a vast section of stars and planets ruled by the mysterious Connatic. The Alastor Cluster may or may not be a part of Vance's Gaean Reach.