The description of the ‘second plague’ is very similar to a story told by Nennius in Historia Brittonum (History of Britain, c.830), and by Geoffrey of Monmouth in his Historia Regum Britanniae (History of the Kings of Britain, c.1138).
In Nennius’ version, the British warlord Vortigern is unable to build a castle at Dinas Emrys (‘Fortress of Ambrosius’) near Beddgelert in Gwynedd because of an underground battle being fought between a red dragon (symbolising the native Britons/Welsh) and a white dragon (symbolising the Saxons). It is foretold by Ambrosius that the white dragon will reign supreme for a long time, but that he will eventually be defeated by the red dragon. In Geoffrey’s version, the story is essentially the same, except that the outcome of the battle is prophesied by Merlin. The telling of the story in ‘Lludd and Llefelys’ is used as the opportunity to explain the place-name Dinas Emrys.
Today, the red dragon is the central feature of the Welsh national flag.