As one of his tasks, the warrior Culhwch had to obtain the Black Hag’s blood to straighten the beard of the giant Ysbaddaden. Arthur and his warriors, on Culhwch’s behalf, set out for the Black Hag’s cave. The first four warriors sent into the cave – Cacamwri, Hwgwydd, Amren the Tall, and Eiddyl the Tall – were beaten half to death.
Finally, Arthur himself jumped into the cave and killed the Black Hag with one well-placed throw of his knife, Carnwennan (Carwenham). Caw of Scotland collected her blood and delivered it to Ysbaddaden.
An Welsh poem in the Black Book of Carmarthen describes a similar battle between Arthur and a hag at Afarnach.
Gwydolwyn the Dwarf | The Legend of King Arthur
Culhwch and Olwen | Late 11th century