Banding together to fight a common enemy
The most significant theme in the novel is banding together to fight a common enemy. Aelin, the novel's main character and protagonist, must band together with people in the realm to fight against an evil, incredibly violent king named Erawan. Alone, Aelin is not able to go head-to-head with Erawan. In a small group, she isn't able to mount a meaningful fight against him. However, it's only when she bands together with friends and other unexpected people (including Manon Blackbeak, the leader of a clan of powerful witches that has traditionally been isolated) that Aelin is able to mount an adequate defense against Erawan. Each of the people Aelin fights with is vastly different; however, they have one, common goal: to defeat Erawan, who threatens them all.
Power
The dynamics of power play a pivotal role throughout Empire of Storms. Aelin's journey from an assassin to a queen highlights her adaptiveness and her interest in keeping and further proliferating power. However, in that sense, the novel also shows the responsibility and burden power brings to those who wish to have it (and those who lust for it). As she prepares for war and seeks out allies with similar interests and goals, Aelin acts out of her duty (and, to a lesser extent, her personal interests) to the kingdom she knows and loves. Aelin is an example of someone using her power for good; however, others in the novel, like the menacing Erawan, are potent examples of people who use power for their own personal interests, which is bad in almost every situation.
