The Circle Trilogy by Nora Roberts
Morrigan's Cross, Dance of the Gods, and Valley of Silence
Hoyt Mac Cionaoith is grieving. His brother Cian has been killed, and worse, changed into a demon. Because Hoyt is a sorcerer, his grief has conjured up a raging storm. The goddess Morrigan appears to him in a vision--telling him the creature his brother has become is a vampyre and that Hoyt is part of a charmed circle of six needed to defeat the vampire queen Lilith. She sends Hoyt from his home in eleventh century Ireland to twenty-first century New York City, to find Cian, who will also need to be part of the circle, which is to include a sorcerer, a witch, a warrior, a shape-shifter, a scholar, and the damned one.
When the group has assembled, (Hoyt, Glenna, the red-haired witch, Blair Murphy, a vampire slayer descended from Hoyt and Cian's sister, Larkin the shape-shifter, cousin of Moira, scholar and future queen of the mythical Geall, and Cian) they must learn to hunt and kill the demons before Samhain, when they will travel through the Dance of the Gods to the mythical Geall and fight Lilith's army at the Valley of Silence.
Nora Roberts' trilogies have been leaning more and more to the supernatural. It isn't something I would have picked up by any other author, but she tells a compelling story. People who enjoy her trilogies will probably enjoy this one, if they can get past the idea that they are reading "vampire books". As in other trilogies, there is plenty of steamy romance and suspense, and I found them hard to put down.