This is book 6 in the series, and as such, not the best place to start reading. If you’re really interested in the series, start at the beginning with Angels’ Blood.
During a terrifying five minute time period, birds and angels start plummeting from the sky, leaving almost half of the archangel Raphael’s forces incapacitated. Shortly after, his consort, the Guild Hunter Elena, discovers a mysterious and highly infectious disease that impossibly enough only targets vampires. All over the world, other archangels are developing new and terrifying powers that could be used for further warfare, while Raphael so far seems only to be developing healing abilities. Is it his relationship with Elena that’s keeping him from becoming more powerful?
Elena and Raphael have to discover which of the other archangel or angels are causing the attacks, and figure out a way to stop them before New York is lost.