Shout-Out: Skyborn by David Dalglish

Six islands float high above the Endless Ocean, where humanity's final remnants are locked in brutal civil war.

Their parents slain in battle, twins Kael and Brenna Skyborn are training to be Seraphim, elite soldiers of aerial combat who wield elements of ice, fire, stone and lightning.

When the invasion comes, they will take to the skies, and claim their vengeance.