Bran Fendiged (or Bendigedfran, Bran the Blessed) was king of Prydein (Britain), son of Llyr (yes, King Lear. In the Mabinogion, he possesses a cauldron of rebirth, which he gives to his brother-in-law Matholwch. Matholwch goes on to abuse Bran's sister Branwen, which starts a war between Britain and Ireland. In this war, Bran is wounded and has his head cut off. This magic head then stays alive for eighty years before the enchantments end (see Branwen uerch Llyr for the full details), at which point the Seven survivors of Prydein return to Britain and bury the head at White Hill--today Tower Hill at the Tower of London--as a talisman of protection against invasion by outside sources.
The unearthing of this head--the unfortunate disclosure--was done by King Arthur as a way of saying that he could protect the island without the divine help of his otherworldly ancestor Bran. This then leads to the Saxons taking over after his death.
The dragons buried by Lludd and Llefelys are told of in the Mabinogion's tale of Cyfranc Lludd a Llefelys. Two dragons were warring in Britain, and were discovered living at the center of the island--here determined as Oxford--and so were buried deep underground at Dinas Emrys/Dinas Ffaron near Snowdonia.
About four hundred years later, Vortigern/Gwrtheyrn was building his fortress at Dinas Emrys, when each time they tried to build, the tower would collapse. According to Nennius, Vortigern was then advised to sacrifce a fatherless boy, and sprinkle the blood on the rocks as an appeasment. Nennius calls the boy Ambrosius--which is the Latin form of Emrys--while Geoffrey of Monmouth identifies him with the young Merlin, born of a woman and the devil. Merlin tells Vortigern that the towers keep falling because of the dragons buried undernieth. Vortigern has the dragons released. There are two--a red and a white--and the red defeats the white. Merlin tells Vortigern that the red dragon represents the Britons (hense the Red Dragon of Wales on the Welsh flag), and the white represents the Saxons. So, the Britons would defeat the Saxons, with whom Vortigern had a special treaty, selling out his country to the invaders. Now, the dragon represented first Uther Pendragon, and then his son King Arthur. This was all bad news for Vortigern.
This tale is recounted in Nennius' history, Geoffrey's History of the Kings of Britain, and in the late-English Renaissance play The Birth of Merlin, or, the Childe Hath Found His Father.