Heavenly Bodies meets Divine Rivals in this star-lit romantasy about a girl who, after trying to end the world, now has to save it. 'A dark and simmering supernova of a story' AMANDA CONNOLLY 'If you loved Throne of Glass and Fourth Wing, you will devour What Makes a Monster!' ROMINA GARBER She tried to end the world...now they want her to save it. In Velandor, where magic is ruled by the constellations, Celestine Savareau rots in prison after a failed attempt to end the world. All she wants is destruction and domination. When she is offered a freedom deal, Celestine has little choice but to accept, even if it means going after her ex-lover and accomplice Aire. Alone, he is trying to complete their monstrous plan. So Celestine joins the Nightguard, the protectors of Velandor, even as she plots to reunite with Aire and finish what they started. Nightguard Kivessin would rather die than work with Celestine. His mother was killed during her arrest, and he's not about to forgive and forget. But when he learns that helping Celestine means taking her down if she goes rogue, he manages to set his razor-sharp morals aside. As they commence training, Celestine must exploit her relationship with Aire to stay one step ahead. If she fails, the world will end-and she'd hate to die without exploring the undeniable tension which lingers between her and Kivessin. In doing so, they discover something neither of them wants to acknowledge: she might not be the monster they thought...