The Mortal Phantom by Vincent Gilvarry

Chaos follows when a Master Arcanist constructs a Black Tower to drain the power of the celestial realm of Sanctus. As shadow-fire and dark arcana choke the skies, an improbable coalition rises to fight back in the form of a banished prince, a fierce warrior, the celestial heir to the throne, and Elias Thorne, a master alchemist. Elias is a meticulously aware and pragmatic guardian who views the world through the lens of equations, reactions, and logic, and in The Mortal Phantom, he battles to save the love of his life in an epic tale in which war, alchemy and spirit magic intertwine.