[3] Myrddin Wyllt's legend closely resembles that of a north-British figure called Lailoken, which appears in Jocelyn of Furness' 12th-century Life of Kentigern.

[citation needed] In the forest he ruminates on his former existence and the events of the Battle of Arfderydd, where Riderch Hael, King of Alt Clut (Strathclyde) slaughtered the forces of Gwenddoleu ap Ceidio, and Myrddin went mad watching this defeat.

[5] This battle, the subsequent assassination of Urien Rheged and the defeat of the Gododdin at Catraeth are cited as reasons for the collapse of the alliance of early British kingdoms in the north before the Angles, Scots and Picts.

The first, “Merlinus Ambrosius” (the Arthurian Merlin), identified by Giraldus Cambrensis as Myrddin Emrys —the Welsh form of Ambrosius—, who was found at Carmarthen and prophesied before Vortigern.

After some hesitation, the saint grants the madman's wish, and later that day the shepherds of King Meldred capture him, beat him with clubs, then cast him into the river Tweed where his body is pierced by a stake, thus fulfilling his prophecy.

However, when Britannia was a Roman province, Carmarthen was the civitas capital of the Demetae tribe, known as Moridunum (from Brittonic *mori-dunon meaning "sea fort"), and this is the true source of the town's name.

The tradition was apparently shared with Cornish literature, however only a single Latin translation of a lost Cornish-language original Prophecy of Merlin exists in the Vatican library by John of Cornwall.

In this work, however, he constructed an account of Merlin's life that placed him in the time of Ambrosius Aurelianus and King Arthur, decades before the lifetime of Myrddin Wyllt.

Geoffrey later wrote the Vita Merlini, an account based more closely on the earlier Welsh stories about Myrddin and his experiences at Arfderyd, and explained that the action was taking place long after Merlin's involvement with Arthur.