Bloody Mary, is a well-known urban legend, so variants on its origin and invocation exist many, this time it is said that:
Back in the year 1720, a nine-year-old girl, named Mary, was left alone at home; his parents went out to make a living on a nightly journey. Leaving the little girl illuminated only by the dim light of three old candles.
She was accustomed, so she quietly set about playing with her worn rag doll in the gloom. Near midnight, some noises were heard at the door, believing that their parents had returned, looked out of the window, but; there was no one!. He then went to bed, then again the noise was heard, she ran to the bathroom to hide, but when she looked at the mirror, her face was filled with terror, because out of the shadows came a strange subject, squeezing in his hands huge knife
Little Mary's death was terrible; bled ... her blood was used as ink, to write on the walls: "She died at 00:00 because she was alone."
Now, the ghost of the child can be invoked at the time of his death 00:00, putting before the mirror three candles, as those that illuminated her, and repeating his name nine times, as the age he had when he died ... then she appears to assassinate you in the same way that they did with her, and end up bathing in your blood trying to recover all that lost, the day she died.
This last act, that realizes with the victims is the one that has gained him the name of Bloody Mary.
images from google