City of Heavenly Fire by Cassandra Clare

In this dazzling and long-awaited conclusion to the acclaimed Mortal Instruments series, Clary and her friends fight the greatest evil they have ever faced: Clary's own brother.

Sebastian Morgenstern is on the move, systematically turning Shadowhunter against Shadowhunter. Bearing the Infernal Cup, he transforms Shadowhunters into creatures out of nightmare, tearing apart families and lovers as the ranks of his Endarkened army swell.