The Seal of Yueh Lao

A Haliverse fantasy: A legacy from the Eldritch past...
Asenath Merrill, sixteen years old, spends her summers studying witchcraft in the village of Chorazin and her nights traveling the uncanny kingdoms of the Dreamlands.
It's all perfectly ordinary, if it so happens that you belong to one of the secretive cults that worship the Great Old Ones, your mother comes from Innsmouth and has tentacles for legs, and your grandmother is the Black Goat of the Woods herself.
When Asenath encounters a mysterious girl in the stone circle atop Elk Hill, however, her prosaic existence begins to stretch and blur into patterns she must struggle to master.
A century before, a family tragedy in the little Massachusetts town of Dunwich spun out of control and nearly plunged the world into chaos. Four centuries before that, armed men came to the Norse settlements in Greenland and slaughtered every person they could find, leaving a legacy that still troubles the family of Asenath's new friend. A secret from the ancient world connects those events with the girl.
Asenath will need all her courage and her fledgling powers as a witch in training to unravel the mystery - and open the way to her own unguessed destiny...