A vicious snake that inhabited the Mournful Mound and killed or maimed many men. Warrious sought the serpent because it had a magical stone in its tail which caused its bearer to receive any amount of gold he wished.
Peredur became interested in killing it during his adventures, but upon arriving at Mournful Mound he was forced to fight a number of other warriors who also wished to kill the beast. Finally, he defeated his competitors, marched up to the serpent, and killed it. Peredur then gave the Serpent’s stone to his companion, Edlym Red Sword.
Source
Peredur | 13th century