The Lambton Worm is the titular antagonist of a famous England legend surrounding John Lambton and his encounter with a monstrous dragon-like creature (a wyrm) that would ultimately be defeated by John but at a cost of a family-wide curse that would last for three generation (known as the Lambton Curse).
According to most accounts of the legend John Lambton was a rather revolutionary and rebellious man for his time-period and instead of observing church on a Sunday he went fishing in the River Wear, true to many tales of the era it is often said a passerby warns Lambton that no good could come from failing to observe church on the Sacred Day but he disregarded their warnings (an act of medieval hubris).
Eventually John caught a strange eel-like creature, ranging from no larger than his thumb to a maximum of three-feet depending on the tale, prompted by the ugly appearance of the creature John claims he caught the Devil and tossed the thing down a well despite another warning by the passerby of potential danger.
John, having a change of heart, decides to make amends for his previously irreverent and sacrilegious behavior, and joins the Crusades - leaving his home land for an extended time as a result.
During John's long departure the "Worm" grew to gigantic size and the well became poisoned, the monstrous creature started to lay waste to local livestock before coiling itself around a hill (having become truly monstrous) - at which point its malicious activities grew worse as it threatened local villages by eating sheep, preventing cows from giving milk and snatching away small children.
The beast made its way to Lambton Castle and forced John's father to appease it with vast quantities of milk - attempts by brave villagers and passing knights to slay the beast only resulted in their demise and the monster's wrath as it uprooted trees and devastated its nearby surroundings by using said trees as horrific clubs.
Seven years of terror continue before John returns and sees how his father's estate was in ruins, prompting him to slay the "Worm" - he would be driven even further to slay the creature after seeking advice from a witch, who revealed to him that the "Worm" was the same creature he had thrown the well all those years ago.
Luckily the witch also gave John instructions on how to kill the beast, telling him to cover his armor in spearheads and fight the "Worm" in the River Wear - she also warns him he must kill the first creature he sees after slaying the "Worm" or face a terrible curse.
John thus set out, giving instructions to his father that once he had slain the "Worm" he would blow his hunting horn three times, at which point his father was to release his hunting hound for him to slay and thus evade the curse.
A great battle was waged between John and the "Worm" in which he hacked away at the monster and the river washed away the pieces, preventing it from healing - attempts by the "Worm" to crush John failed as it cut itself on his modified armour and ultimately John would stand victorious as the river washed the remains of the "Worm" away for good.
However, in a twist of fate, John would not evade the curse as his father, overjoyed at his son's victory, would rush to meet his son and forget to release the dog first - unable to kill his own father John fell victim to what would be known as the "Lambton Curse" - the final vengeance of the terrible "Worm".