A Blood Crystal is a magical artefact that allows its wearer to take on the physical appearance of anyone whose blood the crystal touches.
Two thugs, Dagr and Ebor, were put in the stocks by Arthur Pendragon after they initiated a bar brawl in a tavern because the barmaid Mary refused to give Dagr some money. They wanted revenge on the prince and later visited the sorcerer Cylferth, who gave them the Blood crystals and the Stulorne Blades which appear blunt to the eye but in reality were razor sharp. Cylferth chanted this spell on the crystals: "Þece treowee andwlitan heora fram gesiht eallra!"
After killing the sorcerer, they ambushed two knights: Sir Oswald and Sir Ethan, who were going to take part in a tournament in Camelot, and killed them. Then they took on their physical appearances with the crystals. When they arrived in Camelot they faced Arthur and almost defeated him in the tournament using the magical blades, but didn't due to the intervention of Gwaine. After the fight, Gaius removed the crystals to reveal to Uther the true identities of the knights (Gwaine).