Crimson Death by Laurell K. Hamilton

When a series of brutal vampire murders strikes Ireland, Anita Blake is drawn across the Atlantic at the request of her lover and master vampire, Jean-Claude. What begins as a diplomatic investigation quickly turns into a nightmare as Anita confronts an ancient evil that defies her understanding — a force neither vampire nor human. Trapped between the demands of supernatural politics and her own rising power as a necromancer, Anita must navigate betrayal, blood ties, and forbidden desire. Crimson Death plunges her into a world where every choice exacts a deadly price.