No Greater Sorrow by L.B. Black

As war looms on the horizon, Alejandra Ruiz and her best friend Violet must undergo a series of deadly Trials to secure their places as Dark Saints. Thrust into these perilous challenges with barely any training, Aleja is forced to rely on her burgeoning magical abilities to survive.
But the stakes are even higher than they appear. The Knowing One, a being feared by mortals and immortals alike, has fallen under a deadly curse—a curse that Aleja herself is responsible for.
As the curse weakens him, their enemies grow stronger. A defector brings news of a terrifying plan: to capture Death itself and use it to destroy everyone in the Hiding Place once and for all.

To save both the Knowing One and his realm, Aleja must face treachery, passion, and betrayal, all while harnessing her own dangerous magic.