This is more of Bloody Mary's true story than anything else. So, she was originally Queen Mary Tudor I of England. She was supposed to get married, but was refused at the last moment. While she was in the bathroom putting on makeup, the ex-groom's father set the bathroom on fire and locked her in.
She was looking straight at the mirror when it happened. Believers in the story say her shadow (and soul) was sucked into the mirror to save the body they haunted; others say she died at that moment, as no mortal can survive a fire.
Now we rewind. Before she was to get married, she burnt approximately 300 people at the stake. Everyone says she did that because those burnt weren't Christian people. Some even say she burnt a pregnant woman.
Believers say she haunts mirrors. The ritual they go through to summon her is going into a dark bathroom with no escape except a closed door. You must then turn and say her name thrice. That apparently summons the ex-Queen.
Others that know more about Mary Tudor than Bloody Mary say that she hanged her sister. Untrue. Her sister, Queen Elizabeth I of England, lived much longer than Mary. After that fact, they say Mary committed suicide, which is the most likely answer.
One thing is certain: The ritual is real. I've tried it twice, and summoned the ex-Queen twice.