If contemporary accounts are to be believed, the Countess Elizabeth Bathory is one of the most prolific serial killers of all time. Description from theoldreader.com. I searched for this on bing.com/images
10 Terrifying Historical Figures You've Never Heard Of
Countess Bathory - Countess Elizabeth Báthory de Ecsed (Báthory Erzsébet in Hungarian, Alžbeta Bátoriová in Slovak; 7 August 1560 – 21 August 1614) was a countess from the renowned Báthory family of Hungarian nobility. Although the number of murders is debated, she has been labeled the most prolific female serial killer in history and is remembered as the "Blood Countess."