Cordelia Carstairs seems to have everything she ever wanted. She's engaged to marry James Herondale, the boy she has loved since childhood. She has a new life in London with her best friend Lucie Herondale and James's charming companions, the Merry Thieves. She is about to be reunited with her beloved father. And she bears the sword Cortana, a legendary hero's blade.
But the truth is far grimmer. James and Cordelia's marriage is a lie, arranged to save Cordelia's reputation. James is in love with the mysterious Grace Blackthorn whose brother, Jesse, died years ago in a terrible accident. Cortana burns Cordelia's hand when she touches it, while her father has grown bitter and angry. And a serial murderer is targeting the Shadowhunters of London, killing under cover of darkness, then vanishing without a trace.
Together with the Merry Thieves, Cordelia, James, and Lucie must follow the trail of the knife-wielding killer through the city's most dangerous streets. All the while, each is keeping a shocking secret: Lucie, that she plans to raise Jesse from the dead; Cordelia, that she has sworn a dangerous oath of loyalty to a mysterious power; and James, that he is being drawn further each night into the dark web of his grandfather, the arch-demon Belial. And that he himself may be the killer they seek.
- Cordelia Carstairs — a young Shadowhunter about to enter into a false marriage with the man she secretly loves. She is the bearer of the sword Cortana, that burns her when she touches it, and must find a murderer who's killing Shadowhunters.
- James Herondale — the man Cordelia is engaged to and grandson of the Prince of Hell Belial who struggles with his demonic abilities.
- Lucie Herondale — Cordelia's soon to be parabatai who is hiding a secret from everyone she loves—she can control ghosts and is on a necromantic mission.
- Matthew Fairchild — James's parabatai and Cordelia's close friend who is struggling with a mysterious incident from his past.
- Thomas Lightwood — after being helpless to stop his sister's death a few months before, Thomas is determined to find this killer on his own.
- Christopher Lightwood — an inventor working on developing a new way of sending and receiving messages instantaneously and a gun that works on demons.
- Grace Blackthorn — the adoptive daughter of the recently arrested Tatiana Blackthorn hell-bent on bringing her brother back to life.
- Anna Lightwood — the group's watchful eye and lively friend who is wishes to help her friends but is also fighting off the advances of an ex-flame who broke her heart.
- Ariadne Bridgestock — the Inquisitor's daughter determined to win back the woman she loves, Anna.
- Alastair Carstairs — Cordelia's brother that is trying to make amends for having hurt Thomas and Matthew as adolescents while helping his pregnant mother and dealing with the return of his alcoholic father.
- London Enclave — governing body of the local Shadowhunters, which consists of Will and Tessa Herondale, Cecily and Gabriel Lightwood, Sophie and Gideon Lightwood, the Consul Charlotte Fairchild and her husband Henry Fairchild, and the Inquisitor Maurice Bridgestock.
- Jessamine Lovelace — the ghost guardian of the London Institute.
- Jesse Blackthorn — Grace's dead brother—and ghost—that she and Lucie are trying to resurrect.
- Collector's first editions: These contain 10 paper-cut silhouettes of the main characters—Cordelia, James, Lucie, Matthew, Christopher, Thomas, Alastair, Anna & Ariadne, Grace, and Jesse—by Kathleen Jennings and a short story entitled "One Must Always Be Careful of Books" featuring Brother Zachariah and Magnus Bane on a mission in the Spiral Labyrinth.
- Waterstones exclusive edition: This special hardback edition contains an extra short story entitled "Fever". It will feature the scene from Chain of Gold where James gets scalding fever from Cordelia's perspective.
- Litjoy Crate exclusive: Those who pre-ordered Chain of Iron through Litjoy Crate received an exclusive letter written by Cassandra Clare from one character to another.
- Though the majority of the action takes place in 1903, the narrative moves back and forth in time, quite similar to that in Chain of Gold, with chapters subtitled "Days Past". However, rather than give background on multiple charcters, these flashbacks only follow the history of a specific character—Grace Blackthorn.
- The death's-head moths on the cover are meant to signify Lucie's necromantic abilities.
- This book is told from several characters' point of view; namely Belial, Cordelia, James, Lucie, Matthew, Christopher, Thomas, Alastair, Ariadne, Grace, Malcolm, Will, and Tatiana.