According to tales, Mad Lady Lothston cooked children, which she captured by sending a giant bat - the sigil of House Lothston - out of the castle of Harrenhal, bathed in blood, and served feasts of human flesh.
In the books
In the A Song of Ice and Fire novels, Mad Danelle's backstory is the same. She ruled Harrenhal during the reign of Aerys I and was a loyal vassal of the Iron Throne. She had long red hair and wore tight-fitting black armor.
The Lothstons held Harrenhal during the time of the Tales of Dunk and Egg prequel novellas, with Mad Lady Lothston herself appearing in the third story in the series, The Mystery Knight. Thus she might appear in any potential adaptation of the prequels.
The legends about Mad Lady Lothston bathing in the blood of slain children are loosely similar to the real-life seventeenth century Hungarian noblewoman Elizabeth Báthory.