Shadow Reigning: An Urban Fantasy Adventure. Shadow Blade Series, #6

Now that we're working with my father and brother, life feels so surreal. The vampires and my father have reached an accord, and are coexisting in the Underground. We're headed into autumn and life has been fairly routine the past few months. But when Orik comes to work with a problem, we have to step in. His sister-in-law, Olga, who lives on Whidbey Island, has vanished. Olga's husband and children are frantic.
So we pack up--including Vaurice, who insists on helping--and head out to the island. Once there, we discover that Olga found an ancient relic--a Viking horn. Shortly after she found the horn, odd creatures appeared on the island, and they seemed focused on Olga's house. We discover she opened a door for some of the darkest creatures from the Norse pantheon to enter our realm and they abducted her.
Now, we must race against time to rescue her, before she's dragged into the darkest realms of Hel.