You will receive this quest at the very end of the Ladies of the Wood quest, when the crones tell you about their meeting with Ciri. The crones, who found exhausted Ciri when she came to Velen, took care of her, but not out of a good heart, only for their own benefit and to hand her over to the Wild Hunt. When Ciri discovered what is going on, she decided to run away from them.
When Ciri arrived in Velen, she found herself in the very heart of the notorious Crookback Bog. Exhausted, she sought shelter, but instead found herself in the clutches of three demonic sisters known to the local peasants as the Ladies of the Wood. Ciri quickly realized how poorly that name fit these bloodthirsty hags and fled. She escaped with her life by a hair's breadth, but still had a long and dangerous trek through the marshes ahead of her.
