She Who Knows is a book in The Witcher 3: Wild Hunt.
Folk say they were four at first. The Mother, She-Who-Knows, the Lady of the Wood, came here from a faraway land and, since she suffered terribly from loneliness, she made three daughters out of dirt and water.
A long, long time ago the Mother was sole ruler of all of Velen. Her daughters brought her the people's requests and served as her voice. Each spring, sacrifices of grain, animals, and men were made to the Lady of the Wood on her special night. Yet as the years passed, the Lady of the Wood slipped deeper and deeper into madness. Her madness eventually spread over the land - men took to abandoning their homes and setting out into the bog, where they became food for beasts. Before long, Velen was drowning in blood.
The daughters saw their land nearing destruction and took it upon themselves to save it. When spring came once more, and with it the night sacrifices, they killed their mother and buried her in the bog. Her blood watered the oak atop Ard Cerbin, and from then on the tree grew wholesome and hearty fruit for the people. As for the Lady's immortal soul, it refused to leave its beloved land, and so the sisters imprisoned it. To this day it lies trapped beneath the Whispering Hillock, where it thrashes about in powerless rage.