Red dragon and white dragon

When Vortigern tries to build a castle there, every night, unseen forces demolish the castle walls and foundations. Vortigern consults his advisers, who tell him to find a boy with no natural father, and to sacrifice him. Vortigern finds such a boy (probably young Merlin), but on hearing that he is to be put to death to solve the demolishing of the walls, the Merlin dismisses the knowledge of the advisors. The Merlin tells the king of the two dragons. Vortigern excavates the hill, freeing the dragons. They continue their fight and the red dragon finally defeats the white dragon. The boy tells Vortigern that the white dragon symbolises the Saxons and that the red dragon symbolises the people of Vortigern