Blood Sacrifice

This ritual was usually performed in the days of the Old Religion only by the most powerful High Priestesses on the Isle of the blessed. During Samhain Eve, at the strike of midnight, when the veil that separates the world of the living from the Spirit World (or Underworld) is at its thinnest, a High Priestess would spread an altar, dedicated to the Ancient Gods, with a victim's blood and chant the words of power of the Old Religion. By doing this, probably depending on the power of the Priestess and on her intentions, she would pierce the veil and release the voices of the dead, the Dorocha, gaining magical power from them. The goddess of the winter and of the dead, The Cailleach, is related to this ritual.

A year after the Fall of Camelot, Morgana took the dying Morgause to the Isle of the blessed. She had spent the year looking after her sister and studying the Art. Morgana had finally become a High Priestess and gained the power to perform the blood-sacrifice. She sacrificed Morgause, who didn't want to be part of this world anymore, and used her powerful magic to break the veil and release the Cailleach and the Dorocha. Before laying down on the altar, Morgause gave Morgana a dagger with which the Witch stabbed her (The Darkest Hour).