Beneath the Burning Sea by Amanda Bouchet

SHE’LL BURN BRIGHT ENOUGH TO RIVAL GODS

Separated from everyone they know and love, sworn warriors (and warring allies) Bellanca and Carver are on a gods-given mission: travel to the famed realm of Atlantis, fight through every danger that awaits them, and find the Shard of Olympus in order to end Atlantis’s generations-long magical punishment—all in Zeus’s name.
Zeus needs an army if he has any hope of stopping Olympus’s bloody coup, and headstrong fire Magoi Bellanca knows it’s her duty to deliver one no matter the cost…to herself, to her own growing magic, or to the man she’s come to desire with all the flames in her heart.
He left everything behind in order to guard her back as she faces down her destiny, and Carver swears that together, they can overcome any obstacle. But finding the shard is no easy task, and it’s not long before Bellanca and Carver are dragged into the center of a deadly game of gods…one not even their combined hearts may be strong enough to survive.