Mercedes Lackey 
The Gates of Sleep 

After the sudden death of her birth parents, Marina Roeswood meets her Aunt Arachne, who is to be her new guardian. Slowly Marina realizes that her aunt is the embodiment of the danger her parents had been hiding her from in the backwoods of Cornwall. But can Marina unravel the secrets of her life in time to save herself from the evil that had been seeking her for nearly eighteen years?

About the author

Mercedes Lackey is the New York Times bestselling American fantasy author behind the Heralds of Valdemar series, The Elemental Masters series, the 100 Kingdoms series, and many more. She has published over 100 novels in under 25 years.