Chain of Iron

Cordelia Carstairs carries Cortana, a legendary sword, and has married James Herondale to shield his name and her reputation. The marriage is one of convenience, and it has an end date. James, for his part, is in love with Grace Blackthorn, whose brother Jesse died years ago.

Their private troubles are pushed aside when a killer begins murdering Shadowhunters in London's East End. The weapon he uses suggests something far stranger than an ordinary murderer.

Cordelia and James set out to find him with James's sister Lucie and their friends, the Merry Thieves. Each of them keeps a secret from the rest. Matthew Fairchild struggles with his drinking and a poor opinion of himself. Alastair Carstairs and Thomas Lightwood grow unexpectedly close, and Lucie hides what she is able to do.

The book is the second volume of The Last Hours, a Shadowhunters series whose events take place in London and Paris during 1903. It follows Chain of Gold and is followed by Chain of Thorns, and it picks up directly where the first volume stops, so it is best read in order. The investigation runs alongside a marriage that has a deadline.