Shadow Blood. Ancient Legends, #6

Only a few months ago, Amber Reed would have called herself an ordinary girl in an ordinary world.until she moves to Scotland and is pushed into a world she never knew existed by the sexy and daring McAllister brothers, unable to withstand its perils. Aidan McAllister, the irresistible leader of the Morganefaire brethren, has what he's always wanted: the girl of his dreams. His love for Amber is getting stronger, but for all his wants and needs, there's a price to pay.
As a mystery draws them deeper into the heart of an intricate organization ruled by shifting allegiances, Amber realizes she'll have to choose between her one true love and doing the one thing she swore she'd never do: break the rules by taking someone's life.