The White Road. The Nightrunner Series, Book 5

Dissolute nobles, master spies, and the unlikeliest of heroes, Alec and Seregil have survived exile, treachery, and black magic. But the road that lies ahead is the most hazardous they've ever traveled. For with enemies on all sides, they must walk a narrow path between good and evil where one misstep might be their last. Having escaped death and slavery in Plenimar, Alec and Seregil want nothing more than to go back to their nightrunning life in Rhíminee.
Instead they find themselves saddled with Sebrahn, a strange, alchemically created creature-the prophesied "child of no woman." Its moon-white skin and frightening powers make Sebrahn a danger to all whom Alec and Seregil come into contact with, leaving them no choice but to learn more about Sebrahn's true nature. With the help of trusted friends and Seregil's clan, the duo set out to discover the truth about this living homunculus-a journey that can lead only to danger or death.
For Seregil's old nemesis Ulan í Sathil of Virèsse and Alec's own long-lost kin are after them, intent on possessing both Alec and Sebrahn. On the run and hunted, Alec and his comrades must fight against time to accomplish their most personal mission ever.