- Sophie's backstory. As a parlor maid, she was repeatedly abused by her mistress' son. When he attempted worse things, Sophie brushed him off, causing him to have a fit and disfigured her face, before claiming to his mother that she had threatened him. As a result, Sophie was expelled and forced to live on the streets until Charlotte found and brought her to the London Institute.
- The deaths of Agatha and Thomas. Will in particular laments that he never got around to make amends with Thomas, who was the first person he befriended at the Institute but became distant after Jem's arrival.
- When Tessa pretends that she has committed suicide, Will tearfully begs her not to die.
- Axel Mortmain's backstory. He was raised by loving warlock parents who wanted nothing more than to bring him joy by creating automatons. Unfortunately, because they lived in less enlightened times, his parents were murdered by Shadowhunters simply because of what they were.
- The revelation of Will's apparent curse, which threatens to kill his loved ones if they get close to him. And he got a first hand evidence of this when his older sister died horrifically the day after he received the supposed curse.
- Jem half-angrily half-tearfully punching Will after he and Tessa find him drunk in an ifrit den.
- Tessa is forced to kill Nate and, despite what he had done, laments her inability to save him.
- After Will learns that his curse is a lie, he wastes no time to confess his feelings for Tessa. Unfortunately, he is literally a few hours too late, because Tessa has accepted Jem's marriage proposal. Will then leaves in shame, leaving Tessa to put a hot iron bar on her hand to mark her regret.
- The prologue showing why Aloysius Starkweather became such a Grumpy Old Man: he personally witnessed his granddaughter dying painfully and horrifically after she had a terrible reaction to her first runes. And then he learns that "Adele" is not really his granddaughter, because she was Tessa's mother, meaning his granddaughter did end up die, but only decades after her supposed death. Finally, Starkweather is killed before he gets the chance to meet Tessa again.
- Jessamine's death.
- Will and Jem's heartful talk as the latter is on his deathbed.
- Will discovering, through his parabatai rune, that Jem is gone and promptly breaks down.
- The ending, which is a heartbreakingly accurate representation of MayflyDecember Romance.
- In detail: Tessa and Will marry and live happily, but not for long. Tessa, as a warlock, never physically grows past her late teenager, so she literally cannot grow old with Will. She has to watch as Henry Branwell and Gideon Lightwood predecease her, before Will himself dies of old age. Afterwards, Tessa leaves Britain to move in with Magnus, unable to watch her children die. Considering that she lives all the way to 2008, if she didn't make her decision, she would have watched about five generations of Herondales predecease her.
- The thing is, Tessa's fate is exactly what you'd expect for the lives of warlocks, such as Magnus. Except unlike Magnus, Tessa is capable of having children, but none of them inherit her immortality.
- Even Jem being released from his Silent Brother duties and allowed to live with her is only a temporary respite, since Jem is also a mortal and will predecease her in the long run.