Eleven of the preserved poems, according to Ifor Williams, date from the 6th century and may be ascribed to the historical Taliesin. These indicate that he became court bard to King Brochfael Ysgithrog of Powys around 555, then to his successor Cynan Garwyn, and lastly to King Urien of Rheged and his son Owain mab Urien. Some of the events to which the poems refer, such as the Battle of Arfderydd (c. 583) are known from other sources.
In legend and medieval Welsh poetry, he is often referred to as Taliesin Ben Beirdd ("Taliesin, Chief of Bards"). He is mentioned as one of the five British poets of renown, along with Talhaearn Tad Awen ("Talhaearn Father of the Muse"), Aneirin, Blwchbardd, and Cian Gwenith Gwawd ("Cian Wheat of Song"), in the Historia Britonum, and is also mentioned in Y Gododdin.
Legend has it that, as a child, Taliesin was adopted by Elffin, the son of Gwyddno Garanhir, and that he prophesied the death, from the Yellow Plague, of Maelgwn Gwynedd. In later stories he became a mythic hero, companion of Bran the Blessed and King Arthur. His mythological life-story is found in several late renderings (see below), the earliest being from the hand of Elis Gruffydd (mid-16th century).
According to tradition first recorded in the 16th century, Taliesin was the foster-son of Elffin ap Gwyddno, who gave him the name Taliesin, meaning "radiant brow", and who later became a king in Ceredigion. The tradition states that he was then raised at his court in Aberdyfi and that at the age of 13, he visited King Maelgwn Gwynedd, Elffin's uncle, and correctly prophesied the manner and imminence of Maelgwn's death.
The idea that he was a bard at the court of King Arthur dates back at least to Culhwch and Olwen, perhaps a product of the 11th century, and was elaborated upon in modern poetry, such as Tennyson's Idylls of the King and Charles Williams's Taliessin Through Logres. In any case the historical Taliesin's career can be shown to have fallen in the last half of the 6th century, while historians who argue for Arthur's existence date his victory at Mons Badonicus in the years to either side of AD 500; the Annales Cambriae offers the date of 532 for his death or disappearance in the Battle of Camlann, only a few years earlier than the date of 542 found in the Historia Regum Britanniae.
Bedd Taliesin, a hilltop Bronze Age tumulus in Ceredigion, is a traditional site for his grave; the village of Tre-Taliesin, located at the foot of the hill, was named after the bard in the 19th century. A manuscript in the hand of 18th century literary forgerer Iolo Morganwg claimed he was the son of Saint Henwg of Llanhennock but this is contrary to every other fact and tradition.
John Gwenogvryn Evans dated the Book of Taliesin to around 1275, but Daniel Huws now dates it to the first quarter of the 14th century. Most of the poems in the collection are quite late (around 10th to 12th century and, though some may claim Taliesin as author, are clearly later, while others again are attributed internally to other poets. A few of the "marks" presumably awarded for poems - or at least measuring their "value" - are extant in the margin of the Book of Taliesin.
Of the poems in The Book of Taliesin, twelve are addressed to known historical kings such as Cynan Garwyn, king of Powys, and Gwallog of Elmet. Eight of the poems, however, are addressed to Urien Rheged, whose kingdom was centered in the region of the Solway Firth on the borders of present-day England and Scotland and stretched east to Catraeth (now Catterick in North Yorkshire) and west to Galloway. One poem, a "marwnad" or death lament, was addressed to Owain, son of Urien.
The rest comprises some poems addressing mythological and religious topics as well as a few works such as 'Armes Prydein Vawr', the content of which implies that they were by later authors. Many lack the characteristics, metre and 'poetic tag' associated with the work of the historical Taliesin. Much of this material is associated with the mythical Taliesin.
In the mid 16th century, Elis Gruffydd wrote a mythological account of Taliesin which resembles the story of the boyhood of the Irish hero Fionn mac Cumhail and the salmon of wisdom in some respects, and has many characteristics of creation mythology. The tale was also recorded in a slightly different version by John Jones of Gellilyfdy) (c. 1607). A composite version based on these accounts is given below.
All too soon he heard her fury and the sound of her pursuit. He turned himself into a hare on the land and she became a greyhound. He turned himself into a fish and jumped into a river: she then turned into an otter. He turned into a bird in the air, and in response she became a hawk.
Exhausted, he turned into a single grain of corn and she became a hen and ate him. She became pregnant. She resolved to kill the child, knowing it was Gwion, but when he was born he was so beautiful that she couldn't, so she threw him in the ocean in a leather bag.
Amazed, Elffin asked how a baby could talk. Again Taliesin replied with poetry: