Chthon was Piers Anthonys first published novel in 1967, written over the course of seven years. He started it when he was in the US Army, so it has a long prison sequence that is reminiscent of that experience, being dark and grim. It features Aton Five, a space man who commits the crime of falling in love with the dangerous alluring Minionette and is therefore condemned to death in the subterranean prison of Chthon. It uses flashbacks to show how he came to know the Minionette, and flashforwards to show how he dealt with her after his escape from prison. The author regards this as perhaps the most intricately structured novel the science fantasy genre has seen. It was a contender for awards, but not a winner.
Phthor is the sequel to Chthon, less intricately structured and less complicated in plot, but still quite dark and ugly in theme and detail. Rather than flashbacks and flashforwards, it has a Y outline, with the stem the initial story and the ends alternate futures, neither of which is acceptable to Atons son Arlo. Arlo has his own encounter with a Minionette, and naturally destruction is upon him and all with whom he associates. Write/editor Charles Platt was so impressed with the chthonic setting that he wrote two additional sequels, Plasm and Soma, which are even darker and grimmer.