As told in the Welsh epic the Mabinogion, during the reign of King Lludd ab Beli Britain was beset by three plagues. The first were the Corryaneid who were a race of faery folk. The second was a great shriek heard on May eve which turned milk and caused women to go barren.
The third was that no matter how much food was in the cellars of the king's household, after the first meal no food was left.
Lludd went to his brother Llevelys for advice on how to rid his land of three plagues. For the first plague, he was given some poisonous insects that would only affect the Corryaneid.
The second plague was caused by two dragons fighting (the Red Dragon of the Britons and the White Dragon of the Saxons). The third plague was in fact a thieving giant which invaded his household at night and ate all the food.
Concerning the second plague, Lludd was told to return to Britain and find the exact centre of Britain, which is now the centre of the Carfax crossroads at the heart of Oxford. He was then told to dig a pit and set in it a cauldron filled with the best mead and covered with a satin sheet. This was done, and when the dragons fought on May Eve next, they eventually became exhausted and fell into the cauldron.
There they drank the mead and fell asleep. Lludd gathered up the dragons in the satin sheet took them to Dinas Ffaraon (now called Dinas Emrys) where he buried them. Eventually the dragons were disenterred by King Vortigern, on the advice of Merlin, in order to build the fort whose ruins one can see to this day.