The Soul Thieves by Catherine Fisher
When the evil Snow-Walker Gudrun steals Signi's soul on the eve of her wedding, Kari knows it is his mother's sign to take up her final, most dreadful confrontation - from which only one of them can emerge alive. Together with his stalwart friends, Jessa, Brochael, Skapti and Hakon he sets off to challenge Gudrun in her own land, beyond the rainbow bridge. It's a journey few before have dared attempt -and from which none have returned. . .