Dorocha

"The Dorocha. They are the voices of the dead, my child. And, like the dead, they are numberless."

- The Cailleach to Morgana upon unleashing the Dorocha. The Dorocha or Shadow People are the voices of the dead. It is unknown if all the people who die become Dorocha, or just a portion of them. However, they are said to be numberless. They appear as demonic, screaming skeletons made of smoke. They can kill any mortal by their touch, seemingly freezing them to death, and are only repelled by fire or daylight. According to Gaius, they cannot be stopped by any mortal weapon or magic. In their presence, Merlin could not use his powers to defend himself. However, the Dorocha's touch was not fatal to him. They dwell in the Spirit World, and are separated from the living world by the Veil. The mistress of the Dorocha is The Cailleach, the gatekeeper to the Spirit World.

History
At the time of the Old Religion, on Samhain Eve, the High Priestesses would gather on the Isle of the Blessed and perform a blood sacrifice to the ancient gods, tearing the Veil between the worlds. This would allow the Dorocha to enter the world of the living and the High Priestesses would gain magical energy from them.

When Morgana Pendragon sacrificed her dying sister to open a split in the Veil, the Cailleach and the Dorocha came forward. The Dorocha spread throughout the land and attacked villages before reaching Camelot. According to the Cailleach, Morgana's enemies would rue the day the Veil was torn. If the Dorocha had been allowed to remain in the world of the living, they would have caused massive devastation. The Dorocha were sent back to the Spirit World when Lancelot sacrificed himself to repair the Veil (The Darkest Hour).