Seventeen-year-old Rioyn Orro dreams of protecting his realm of Seivan. It would entail joining Sansyre University’s Order of Soldiers, but his father, a general who finds his son too impulsive, won’t approve his enrollment. Rioyn plans to join anyway; histwin, Ayva, who thinks of herself as a “nerdy book girl,” is already signed up for the Order of Galilei. As each sibling gears up for their orders’ tournaments, their realm suddenly comes under attack. A sinister brother and sister, wielding dark Ascendances (magical abilities), lead the Starless Army, which consists of soldiers whose “lacquered black eyes” indicate apparent mindlessness. Rioyn and Ayva have their own Ascendances, like many in Seivan, but neither has mastered them yet. To restore peace in their realm, they’ll need to find the Iyanndyre Born, a fabled and enigmatic figure. With their loved ones in peril, the twins trek across a mountain range to reach the towering Iyanndyre monolith. Sostman’s novel throws numerous obstacles in the siblings’ way; their journey to the Iyanndyre structure, for example, is rife with life-threatening dangers. However, the characters also struggle with more relatable troubles, such as Rioyn’s craving for his father’s approval and Ayva’s wish that she was popular, like her brother. However, much of this opening installment is deliberately cryptic about key topics, such as who or what the Iyanndyre Born is and how the mysterious Infinite Void relates to Seivan and “the Outer Realms.” The alternating narrative perspectives, primarily the twins’, provide useful insight. Unfortunately, they also reveal Rioyn as a selfish and largely incompetent character; he’s essentially a detriment to the mission, especially compared to the siblings’ traveling companion (and Rioyn’s embittered ex) Falla Kai. An effervescent secondary cast includes villains with startling, well-developed origin stories, as well as Jax Risor, Ayva’s love interest and Rioyn’s best friend. The final act answers a lingering question while also deepening other mysteries.