Themes are the fundamental and often universal ideas explored in a literary work.

The Conflict Between Love and Allegiance 

Throughout the novel, various characters feel torn between their feelings of romantic love for an individual and their allegiance to a group. This theme plays out in the relationships between Kaz and Inej, who repress their affections to focus on the mission, and to a lesser extent between Jesper and Wylan, whose budding relationship must also take a back seat to the priorities of the heist. But the relationship between Nina and Matthias provides the deepest exploration of this theme, as both characters struggle to reconcile their love for each other with their allegiance to their countries.  

Matthias feels especially tortured by his feelings for Nina because, as a Fjerdan, he has been raised to believe that Grisha are witches whose powers are seductive but evil. His mentor, Jarl Brum, has inculcated him with the idea that Grisha are sub-human abominations with no right to exist. Matthias’s personal history gives him further cause to hate Grisha, his family having been murdered by a Grisha army. Yet his undeniable affection for Nina causes him to act in ways that acknowledge her humanity and challenge his deep-seated beliefs. Throughout the novel, Matthias has many opportunities to destroy Nina, but he repeatedly opts to save her instead. Each time he chooses Nina over his country, he struggles to understand his own behavior and loathes himself for betraying his people. In the Ice Court, Matthias must decide between betraying Nina or his country. Ultimately, he chooses Nina and is branded a traitor to Fjerda. Repurposing the oath he once gave to his country, he promises his allegiance to Nina, until death. 

Though Nina is also deeply conflicted, she is more accepting of her feelings for Matthias. As an empathetic person whose powers make her attuned to others’ emotions, Nina can sense the mutual attraction between herself and Matthias early on. However, in forming a connection with Matthias, she allows herself to forget the grotesque realities of the war that his people—and he himself—are waging against her own. She faces this reality when the group arrives in Fjerda and discovers the charred bodies of a group of Grisha tortured by drüskelle. Matthias’s half-hearted condemnation of the killings forces Nina to reckon with the harsh reality of Matthias’s darker side. As Nina moves through the Fjerdan prison and laboratory, she becomes increasingly enraged and sickened by the abuses heaped upon the Grisha. Eventually, she wins Matthias’s heart, but she remains loyal to her people. Even though Matthias begs her not to take the jurda parem, she sacrifices herself to save her people.   

The Power and Danger of Vengeance 

The character of Kaz Brekker illustrates the power of vengeance as a motivating force, as well as the dangers of becoming consumed by revenge. Like most of the story’s main characters, Kaz’s childhood was truncated by an egregious wrong, and he was forced to make his own way in the world. However, Kaz distinguishes himself from others through his singular focus on avenging this wrong. From the moment Kaz emerges from the harbor after Jordie’s death, he is focused on one thing: getting revenge on Jacob Hertzoon, who later turns out to be Pekka Rollins. Kaz builds his entire identity around revenge, strengthening himself so that he can defeat enemies, joining the Dregs so that he can achieve power, and searching ceaselessly for information on Rollins. This thirst for vengeance gives Kaz the strength to survive a childhood alone on the streets, helping him accumulate enough power to crush rivals who are older, stronger, and more experienced. His desire to crush Pekka Rollins, drawing out his punishment before eventually killing him, gives Kaz a sense of purpose and focus that allows him to outwit all his rivals, not just Rollins.  

However, Kaz’s monomaniacal pursuit of revenge comes at a cost. Kaz struggles to connect with other people, maintaining an emotional distance even from those he cares about, like Inej. He shares nothing of his past with others, concealing his origin story as a source of weakness that might be exploited by his enemies. And though he tries to remain focused on his vendetta against Rollins, he remains haunted by the memory of his brother Jordie, who surfaces constantly in his private thoughts and dreams. Because Kaz refuses to discuss his past with anyone, he struggles with a tremendous psychological burden all by himself. During the heist, events that trigger his memories of Jordie cause him to freeze up, faint, and nearly drown. In addition, when he learns that Rollins is at the Ice Court, he deviates from the plan in order to pursue Rollins, putting the entire team in danger. These lapses suggest that Kaz’s obsession with vengeance has skewed his perception to the point that he becomes a danger to himself and others. Ultimately, the heist succeeds not because of Kaz’s legendary cunning and scheming, but in spite of his failures and lack of focus. Overall, his overdeveloped sense of vengeance proves as crippling as it is empowering. 

Reclaiming One's Power in the Face of Trauma 

Most of the novel’s main characters have a backstory that involves a traumatic formative experience that has rendered them powerless and robbed them of their sense of self. The heist presents these characters with an opportunity to overcome their traumatic pasts, reclaim their lost power, and right past wrongs. The stories of Inej and Kaz provide two examples of this theme.  

As a teenager, Inej was taken from her family, imprisoned on a slave ship, and sold into sexual slavery at the Menagerie. The powerlessness and trauma of this experience still haunts her even after she leaves the Menagerie and joins the Dregs. She feels lost in Ketterdam and longs to return home yet feels she can no longer return to her old life. In this state, Inej often seeks Kaz’s attention, but his cold indifference belies his true feelings, making her wonder if she is anything more than an object or a tool to him. Although Inej joins the heist reluctantly, she finds new purpose during her brush with death in the incinerator shaft. She resolves to dedicate her life to bringing slavers to justice, preventing others from suffering her own fate. From this point on, Inej acts courageously, conquering the incinerator shaft and her nemesis, Tante Heleen from the Menagerie. By facing her past and reckoning with her trauma, Inej emerges with a new sense of self-confidence and purpose.  

Kaz’s early life is similarly defined by trauma. He is orphaned at a young age and soon after loses his older brother Jordie, but not before Jordie is duped out of the family inheritance by Pekka Rollins. Kaz nearly dies himself in a sick boat surrounded by corpses, surviving only by using his brother’s bloated body as a life preserver. These traumatic events render Kaz powerless and alone on the harsh streets of Ketterdam. But rather than succumb to fate, Kaz reinvents himself, determined to become powerful enough to avenge his brother’s death. Although Kaz succeeds in amassing tremendous power, he does so by forging a steely exterior to shield himself from the pain of his past. His cold ruthlessness serves him well on the streets, but it also highlights his inability to make emotional and physical connections with other humans. Traumatized by the corpses on the sick boat, he recoils from human touch and considers himself a monster. Although he is aware of his desire for Inej, he represses his feelings, afraid of where they might lead. During the heist, Kaz tries to maintain his distance from Inej, but when she nearly dies, he begins to understand that he needs her. Even so, he is unwilling to share much of his past with Inej, and his meager attempts to express his feelings only succeed in pushing her away. Only when he finally opens himself up to Inej will Kaz be able to face his past and reckon with his loss.