Greatfall is set in Hugh Howey's world of Wool. I don't read fanfiction so I was a little hesitant to pick this up, but Gurley DELIVERED. His story takes Silo 23 and perverts Howey's world into a disconnected deranged dungeon filled with religious fervor and a serious stockpile of crazy. This was a thrill ride from start to finish: not many books are brave enough to open with the sacrificial murder of a thirteen year old girl, but Gurley does it in such a way that you have to know what happens to these people. Would definitely recommend to fans of Wool.