The Lambton Wurm !

The Lambton wurm was caught by a fishermen named John Lambton he threw the monster into a well. He then changed his life around and went on a pilgrimage he was gone for many years and the wurm continued to survive in the well eventually it had grown up and was terrorizing the nearby town, many people would try to slay it but every time they cut it, it's halves would rejoin.

John Lambton returned from his pilgrimage and found that the wurm he had caught so many years ago was causing much havoc, he sought the help of an old witch who told him to make a special armor of blades and fight in the river where he had caught the wurm. John obtained an armor of blades and fought the wurm in the river so that each segment of the wurm would float away.

But there was also a curse along with the wurm, after he defeated it he was told to kill the first person he saw which was his father, Instead he killed his pet dog which did not dispel the curse and every heir of the Lambton family met an early death for the next nine generations.