This ritual of sacrifice 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 ("Eala leofu sweostor þæm gastum befæste ic þe. Alynne þa þeostre þe inne onwunaþ. Onginn dwolma"). 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. Morgana had spent the year looking after her crippled sister and studying the Art. Morgana had finally become a High Priestess and gained the power to perform the blood sacrifice. Morgause told Morgana to sacrifice her, stating "there is nothing left for me here now". She then told Morgana "let my parting be my final gift to you", as by sacrificing herself, the Veil between the worlds would be torn and the Dorocha released. Before laying down on the altar, Morgause gave Morgana a dagger with which the Witch stabbed her.
The Great Dragon told Merlin that in order for the Veil to be opened the Spirit world demanded nothing, only the Cailleach, the gatekeeper to the Underworld, asked for a life to tear and then heal the Veil.