She was coming to get your blood, because she supposedly bathed in the blood of young girls to stay beautiful.
As the urban legend experts at Snopes.com point out, this aspect of the mirror witch was based on a real person, Elizabeth Bathory, the so-called Blood Countess.
A Hungarian noblewoman who lived from 1560 to 1614, she was arrested for torturing and killing dozens of women in order to bathe in their blood. There is evidence that she killed more than 600 victims in all, using hot pokers, an iron maiden, pins, blades, and freezing water. Katherine Ramsland's account of the Bathory legend -- in all its gruesome detail -- is here. Bathory was walled alive in Cachtice, her castle located in what is now Slovakia.
"The Bloody Countess lived sealed in the tomb for three and a half years," according to a travel guide for Slovakia. "Her only company was her victims’ ghosts and the bats that flew in through the lone hole high up on the wall to sleep hanging from the curtains...
All that remains today of Báthory is her legend and her crumbling castle ruin above the village Višòové. Among these ruins is a certain room. It is circular and dug deep into the soil. There are no doorways, no designed entries whatsoever. But high up on the wall is a slit, a lone window which would have permitted a single thin band of light to penetrate the Bloody Countess’s prison.
(12/10/2010 UPDATE: There is a similar legend in France which I have written about here. Enjoy.)