The legend 

claims that the evil woman can be summoned by chanting "Bloody Mary" into a mirror anywhere from three to one-hundred times in a darkened room lit only by a candle. (Thirteen seems to be the most popular number of chants, appropriately so.) The bathroom is the most popular setting to test out the legend, but other dark rooms seem applicable.

After the given amount of chants, the spirit will then appear in a mirror to claw your eyes out. Death will follow. Other variations have her driving you insane or pulling you into the mirror, never to be seen again.

INFO
The legend of Bloody Mary is explored in many different stories and folk tales. Invoking Bloody Mary is a popular game for girls at sleepovers and horror movies. Bloody Mary's impact on popular culture is unmistakable from the historical legends to the many folk stories, and even an alcoholic drink based on the legend.
But is Bloody Mary real?
The short answer is yes, Bloody Mary is real. The long answer is, bloody Mary existed at one point, but no one is certain whether or not the various Bloody Marys of history and the Bloody Mary of legend are one and the same. The answer to that question remains part of the alluring mystery.While you may think you're safe from mean, old Bloody Mary, think again... Legend has it that if you are near a mirror in total darkness, she can still come for you, regardless of whether or not you're trying to call for her.

Who Bloody Mary really is remains a mystery. While there are many versions of this story, most accounts point to a woman named Mary Worth, who was horribly disfigured in a car crash. Some people still tell of a witch who was burned at the stake and has returned for revenge victim of the salam whitch trials but no mary worth was recorded at salam , or it may be the devil himself who comes for your soul.

Some confuse this legend with other bloody marys in history

Queen Mary I
Bloody Mary is the black nickname for Queen Mary I or Queen Mary Tudor, eldest daughter of King Henry VIII and Catherine of Aragon. Like her Spanish-born mother, Mary Tudor was a devout Catholic. She loathed her father's conversion to Protestantism as well as his very public and humiliating divorce of her mother. When Mary ascended the throne upon King Henry VIII's death, she began a campaign to return England to the Catholic Church. During her reign, hundreds of Protestants were tortured, turned out from their homes or burnt at the stake. Queen Mary preferred the methods of the Spanish Inquisition and the very public executions fed into her 'Bloody Mary' nickname. Historians cite Mary Tudor's barren womb and her possible death by the bursting of an ovarian cyst as another source for her bloody moniker. Although Henry VIII executed many more people during his reign than his daughter did, the Protestant reign of Mary's younger sister Queen Elizabeth I far outlasted her own, and history is typically written by the victors.

Countess Elizabeth Bathory
Not often associated with the 'Bloody Mary' legend, some tales maintain that 'Bloody Mary' referred to the Transylvanian noblewoman who lived during the same time period as Queen Mary Tudor. The Countess Bathory is also closely tied to legends of vampires, and is said to be among those that inspired Bram Stoker's original Dracula. So why the "bloody" appellation? It seems that among the Countess Bathory's proclivities was the desire to maintain her youth and beauty. She did this by drinking and bathing in blood. Her exotic and often cruel tastes extended to other areas of her life as well, but it was her murders of many virgin girls that brought her notoriety throughout Europe.
Popular Culture

Could the legend be based on a real story? No one can prove it's not, therefore, the chance remains that Bloody Mary is indeed real and her story is remembered every time a young child whispers her name into a mirror.

It is possible these mirror witch games have their roots in oldtime dividing rituals where unmarred women chanted a ryme into a mirror in a darkend room  on a special night then looking into a mirror to catch a glimps of their bridegroom to be

The concept of mirrors as portals between this world and the next shows up in other belifs like covering a mirror at a funeral (before funeral parlours the dead would be laid in the house for several days and it was believed if the spirit caught a glimps of him/herself in a mirror the ghost would remain in the house as the mirror would trap the soul.