Ashes Beneath the Tree. The Viking Witch Mysteries, #16

Ingrid Torfudottir moved to the North Shore of Lake Superior intending to live with her grandmother while pursuing a career as a book illustrator. Instead, she discovered her true calling as a volva-a Viking witch-serving a village founded in the Viking age by Norse settlers escaping some terrible enemy whose identity was lost to time. Now she lives in the hidden town of Villmark, protected by magic from the perceptions of anyone in the larger world.
But more lands lie beyond Villmark. Lands that extend through places where time moves at different speeds. Lands filled with trolls, giants, and witches lurking in magical towers. Lands, in short, where the older magics still thrive. Such places exist outside of Ingrid's domain as volva under normal circumstances. But when Ingrid's ward Esja gets tangled up with a young man with a dark history, one who lures her out into those wilder lands, what happens next is far from normal circumstances. Someone wants that young man dead.
Someone immensely powerful. But not as powerful as those who ride during these, the longest, darkest nights of the year. The ones that no human can control. Just to see them is to see your own doom. But if your foe wields the old magics, sometimes calling on the oldest magic of all is your only hope.