What happens in city of lost souls?

In "City of Lost Souls," the fifth book of Cassandra Clare's "The Mortal Instruments" series, Clary Fray and her friends embark on a mission to rescue her brother Jonathan, who has been taken by the demon Sebastian Morgenstern. As they navigate the Shadow World, they confront dark forces and face complex relationships, particularly Clary's feelings for Jace, who is under Sebastian's influence. The story explores themes of love, loyalty, and the struggle between light and darkness while delving deeper into the characters' internal battles. Ultimately, the group must make difficult choices to save their loved ones and confront their destinies.