Angels of Istanbul — Alex Beecroft

But when Ecaterina's brother dies of a mysterious new plague, it's clear to Radu that his parents have not been idle. Soon Bucharest is in the grip of an undead epidemic -- a less than ideal time for Ottoman Sultan Mahmud, Wallachia's overlord, to call Bucharest's nobility to assemble their armies in Istanbul for a holy war against Britain.

The Wallachians have long resented their Ottoman overlords, so Radu seizes the chance to eliminate them while also ridding Bucharest of the undead: he leads an army of vampires to Istanbul and sets them to feed on the Turks.

As Radu's demons gut the city of Istanbul, their plans become horribly clear. This is only the start. With the Ottoman armies under their control, the undead are poised to suck the life out of the whole world. Radu, his lover Frank, and Ecaterina are appalled at what they've unleashed. But they may be too late to stop it.