(I would've posted this on 21 August, but I'll probably be without an internet connection on Thursday!)
She is probably the most (in)famous Hungarian noblewoman in history, as the No.1 female serial killer not only in Europe but in the whole world. She allegedly tortured and killed hundreds of girls between 1585 and 1610. After her trial in 1610, she was imprisoned in a walled area of her castle in December 1610, where she died slightly less than four years later. By the way, the 21st of August is the day she was found dead lying on the floor; she may have died earlier.
The most famous tale about the Blood Countess is that she had the girls bled to death so that she could bathe in their blood and retain her beauty. According to the Wikipedia, this is a later addition to her legend, a detail not mentioned at the time of her trial. Still according to the Wikipedia, György Thurzó noted the presence of horribly mutilated dead, dying, and imprisoned girls at the time of her arrest. György Thurzó was the man whom Emperor Matthias had asked to investigate the horrible rumours about the Blood Countess.
However, Emperor Matthias was heavily indebted to the incredibly rich countess, and, for political and religious reasons, he was also opposed to the Báthorys, who ruled large autonomous or semi-independent estates all over Central Europe. Some scholars argue that the whole rumourmongering and trial were the results of a vast conspiracy against the rich and independent-minded countess.
György Thurzó, the man who conducted the investigations and assembled the evidence was Archduke Matthias' right hand man before the latter became emperor. One of the consequences of the trial was that the Emperor never had to repay his debt to her.
Whatever the historical truth, you may use the Blood Countess in two ways in your campaign: as a horrible villain, or even as a vampire-like 'big boss' figure, or as innocent and fiercely independent woman. In this latter case, she could be the patron of the player characters, whose aim would be to thwart Thurzó's schemes in a politically-oriented campaign.
