Blood Sacrifice

"But for the tear to be created would've required a blood sacrifice. To seal it will require another."

- Gaius to Arthur about the Veil

Blood Sacrifices are rituals of human sacrifice used to tear open the veil between the worlds. This ritual must be performed when the veil is at its thinnest, and a second sacrifice is required to counteract the first.

History
In the days of the Old Religion, the High Priestesses would gather together on Samhain's Eve and perform a blood sacrifice to release the Dorocha.

One year after her failed attempt to conquer Camelot, Morgana sought revenge by sacrificing Morgause on the Isle of the Blessed. Morgause lay down on a stone alter, and at the stroke of midnight, Morgana chanted a spell and plunged her dagger into her chest. The ritual tore the veil between the worlds and unleashed the Dorocha upon their enemies.

The Dorocha plagued Camelot for several days before Gaius determined that they could only be banished by repairing the veil, which would require a second sacrifice. Lancelot later accomplished this when he willingly entered the veil on the Isle of the Blessed (The Darkest Hour).

Trivia

 * This ritual must be performed at midnight on Samhain's Eve, when the veil is at its thinnest (The Darkest Hour).
 * According to Kilgharrah, it is the Cailleach rather than the Spirit World who asks for a second sacrifice to counteract the first (The Darkest Hour).