Skyfarer - Joseph Brassey

The Axiom Diamond is a mythical relic, with the power to show the bearer any truth they desire. Men have sought it across the Endless Sky for many centuries, but in vain. 

When trainee sorceress Aimee de Laurent's first ever portal-casting goes awry, she, her mentor, and the entire crew of the skyship Elysium are thrown into the race to find the gem. Opposing them are the infamous magic-wielding knights of the Eternal Orderand their ruthless commander, Lord Azrael, who will destroy everything in his path to acquire the relic. 

Surrounded by enemies, and plunged into a small nation's fight for survival, Aimee and her allies must contend with an unthroned prince, a kingdom-destroying superweapon, and a foe whose past hides secrets darker than perhaps all but the most powerful of magics can reveal...