The walls are covered with mask-like faces, which Arya realizes are the faces of people who die at the House of Black and White.
Arya's blood is used to soften the leathery mask into living skin, which then bonds to her head and becomes her new face. The Kindly Man warns Arya that she will suffer the nightmares of the girl whose face she has taken, but only for a time.
- A Dance with Dragons by George R.R. Martin -