The Siren Depths — Martha Wells

All his life, Moon roamed the Three Worlds, a solitary wanderer forced to hide his true nature -- until he was reunited with his own kind, the Raksura, and found a new life as consort to Jade, sister queen of the Indigo Cloud court. But now a rival court has laid claim to him, and Jade may or may not be willing to fight for him. Beset by doubts, Moon must travel in the company of strangers to a distant realm where he will finally face the forgotten secrets of his past, even as an old enemy returns with a vengeance. The Fell, a vicious race of shape-shifting predators, menaces groundlings and Raksura alike. Determined to crossbreed with the Raksura for arcane purposes, they are driven by an ancient voice that cries out from . . . .The siren depths.