Jenny o' the Woods used to be a woman named Zula who lived in Midcopse. Zula's transformation into a nightwraith came after she was murdered by another villager named Bokhai whom she rejected in love, as she was in a relationship with Ontan. Bokhai chased after Zula with a knife, and either stabbed her or slit her throat, causing Zula to come back as a nightwraith in order to exact revenge upon him, presumably killing him in return. The other residents of Midcopse assumed Zula and Ontan ventured north to seek happiness.
Midcopse's ealdorman, Bolko, posted a notice for a contract on Jenny o' the Woods, which Geralt of Rivia took on. Ultimately, Geralt slayed Jenny o' the Woods one night, about one week after Zula's murder.
Bestiary entry
It is said true love's flame is never extinguished. This sad truth is the reason why Zula of Midcopse, whom an early death had separated from her beloved, was unable to find peace in the next life, and instead returned to haunt her former environs as a nightwraith.
Fighting such an apparition is extremely difficult. A nightwraith will form mirror images of herself to confuse her opponent and aid her in battle. She herself can take on immaterial form, rendering her invulnerable to blows. The best way to force her out of this state is to set a trap with the Yrden Sign, then quickly followup with Igni while she is caught. Most important of all, however, is this: never attempt to fight one in the middle of the night, when the moon hangs high in the sky.
This vengeful wraith might very well have killed every last inhabitant of the village, had not a famous witcher, Geralt of Rivia, appeared in Velen - and been in need of a bit of coin.
Bolko states that Zula and Ontan disappeared about one week prior to Geralt's investigation, however, any amount of in-game time may pass between taking on the contract and actually investigating the matter.
Videos
The Witcher 3 Jenny of the Woods Boss Fight (Hard Mode)
