The Blood Guard is an ordder of warrior priests sworn to protect the High Priests and Priestesses of the Old Religion with their lives. The Blood Guard's symbol is the Rowan Tree that grows at the very heart on the Isle of the Blessed. Morgana and Morgause, when they conquered Camelot, made this symbol their crest. Only the Blood Guards, being the most loyal servants of the High Priestesses, have had the privilege of ever setting eyes on the Rowan Staff, carved from the Rowan Tree. Edit
Uther, during the Great Purge, hunted and killed almost all of them. By the beginning of Series 3, they were thought to have been wiped out completely. However, some of them have survived and continued to serve the High Priests and Priestesses, Morgause and Morgana among them. When Morgana joined Morgause, and returned to Camelot once more, she used a blade marked with the sigil of the Blood Guard to wound a guard. Gaius knew of the Blood Guard and the mark, so he warned Arthur that probably there was a traitor in Camelot. Two of the Blood Guard followed Morgause to Cenred's castle and were ready to protect her when she was attacked by one of Cenred's guards. They also captured Merlin when he was found listening to Morgana and Morgause's conversation in the Darkling Woods (The Tears of Uther Pendragon).