Fortune's Blade. Dorina Basarab, #8

Dory and Dorina Basarab were once one person, albeit an unusual one. They were a dhampir-the hated cross between a human and a vampire that is reviled and feared in both circles. But the human and vampire parts of their nature were recently torn apart by a fey weapon, and now, for the first time in five centuries, they are two separate people. And one of them is being hunted. Dory and her husband, the vampire senator Louis-Cesare, brave the strange and often deadly world of Faerie looking for her other half.
But even finding her proves difficult, and despite their skill and abilities, they struggle to stay alive. And then struggle again with the secrets about their family that their search uncovers-secrets that may give them a fighting chance in the war they are waging against the ancient gods, or may end up destroying two worlds.