City of Lost Souls is a 2012 YA fantasy novel by Cassandra Clare. It is the fifth book in her Mortal Instruments series, which began with 2007’s City of Bones. The series centers around a young girl named Clary Fray, who discovers she is a half-nephilim, or angel, and joins forces with a group of nephilim called Shadowhunters who fight evil. In this installment, Clary discovers Lilith, a demon, has magically bound her boyfriend Jace with her evil brother Sebastian. If Clary tries to destroy Sebastian, she will kill Jace as well. She must either find a way to save him or lose him forever. Cassandra Clare made a name for herself with Harry Potter fanfiction before turning to traditional publication with the Mortal Instruments series and a prequel series, Infernal Devices.