Here's why this is considered the climax:
* Luce's choice: After Daniel is injured by Cam, Luce realizes the only way to save him is to use the power of the "angel blood" within her, a power she hasn't fully understood or controlled. She chooses to use it despite knowing the consequences - her own death.
* The stakes: The stakes are incredibly high. Luce and Daniel have been fighting for their love throughout the book, and this is the ultimate test of their devotion. The potential loss of either is devastating, especially since Luce believes she's the key to stopping the apocalypse.
* The emotional impact: This moment is deeply emotional for both the characters and the reader. Luce's sacrifice is a powerful display of selflessness and love, and it leaves Daniel heartbroken and questioning everything.
* The consequences: The climax sets the stage for the rest of the book. Daniel, now immortal and consumed with grief, must find a way to save Luce and stop the apocalypse. The ending is left open-ended, raising the question of whether Luce's sacrifice was worth it.
While there are other tense and dramatic moments in the book, the climax revolves around Luce's decision to sacrifice herself for love. It's a turning point in the story, one that changes the fate of both characters and sets the stage for the rest of the series.