Book 1 of the Dragon Brigade.
The known world floats upon the Breath of God, a thick gas similar to Earth's oceans, with land masses accessible by airship. The largest of these land masses are ruled by the rival empires of Freya and Rosia. Magic is intrinsic to the functioning of these societies, and is even incorporated into their technological devices. But now a crucial scientific discovery has occurred that could destroy the balance of power—and change the empires forever.
In a world where magic is intrinsic to the fabric of everyday life, two kingdoms, centuries-long enemies, have long sought a powerful magical weapon that will win them lasting dominance. But neither realm is ready when they are both attacked by the Bottom-Dwellers, a bitter people whose own land was destroyed, and who now live only to take vengeance on those they blame for a wretched life in the storm-tossed abyss they inhabit. Using contramagic strengthened by blood sacrifice to attack the world above, they threaten to bring down whole cities, or even the island kingdoms themselves. Freya and Rosia are forced to put aside their age-old conflict to defend themselves, or risk losing everything.