Dinas Emrys (meaning the fortress of Ambrosius) is a rocky outcrop near Beddgelert in North Wales.
According to legend, when King Vortigern fled to Wales to escape Anglo-Saxon invaders, he started to build a fort on the rocks, however, every morning they would return to find the masonry collapsed in a heap. This continued for a while until Vortigern saught the help of a young boy so named Myrddin (Merlin). At first Vortigern planned on sacrificing the boy to appease the restless mountain. Myrddin proposed that instead they should investigate in case something under the mountain that was causing the tremors. On investigation they discovered a pair of fighting dragons, one red and one white. The dragons rushed from the mountain and continued their battle in the sky; the white dragon seemed to have the upper-hand as it drove its opponent to the ground. However, the red dragon kept fighting eventually causing the white dragon to retreat and fly away.
Nennius states that the story was in fact a prophecy; predicting the victory of the British and the eventual expulsion of their Saxon conquerors.