Elizabeth Bàthory: The Blood Countess.

Born into one of the most noble houses in Hungary, Elizabeth later forged a reputation as being the most prolific female serial killers in history, although the number of murders is debated, and is remembered as the “Blood Countess.”

According to testimonies at her trial:

• One day, infuriated, Elizabeth struck one of her servant girls so hard that some blood dripped from her face onto Elizabeth’s hand and she immediately thought that her skin took on the glowing freshness of her young maid.Elizabeth believed she had found the secret of eternal youth. After this, women were abducted and hung upside down, while they were still alive and their throats were slit to prepare Elizabeth’s bath. All this was done in an effort to preserve her beauty.
• Another accomplice claimed they tied the hands and arms of victims very tightly with Viennese cord, they were beaten to death until the whole body was black as charcoal and their skin was rent and torn. One girl suffered more than two hundred blows before dying.
• Dorko [another accomplice] cut their fingers one by one with shears and then slit the veins with scissors.
• It has also been claimed the Countess used red-hot pokers from the fire, shoving them into the mouth of some hapless girl, or up her nose.
• Elizabeth’s childhood nurse, Ilona Joo, also testified and claimed the Countess had placed her fingers into the mouth of one girl and pulled hard until the sides split open. She had also stabbed them all over with needles, making them bleed, or had torn open their flesh with sharp pincers. She liked to slit open the skin between their fingers.
• The third accused accomplice added that the Countess liked to apply a red-hot iron to the soles of the girls feet.
• Sometimes, depending on the Countess’ whim, her favorite girls got the worst treatment. One had been forced to strip a piece of flesh off her own arm and feed it to a male servant. A few were shoved into small cages full of spikes.
• She also reportedly enjoyed having cold water poured over naked girls, while they were outside in the cold and snow, until they froze to death and having naked girls smeared with honey and then forcing them to stand outside for hours enduring bites from insects. 

All in all, she was convicted of 80 murders however she is suspected of carrying out over 650.

Source: (X) and the Encyclopaedia of Serial Killers by Michael Newton.