Whence come night and flood?
How they disappear?
Whither flies night from day;
And how is it not seen?
These lines are taken from a poem by Taliesin, a Welsh poet who lived roughly between 534 and 599 AD. His poetry has survived in the medieval Red Book of the Hergest, and The Book of Taliesin, found here: http://www.llgc.org.uk/index.php?id=bookoftaliesinpeniarthms2.
“It is this manuscript which preserves the texts of famous poems such as ‘Armes Prydein Fawr’, ‘Preiddeu Annwfn’ (which refers to Arthur and his warriors sailing across the sea to win a spear and a cauldron), and elegies to Cunedda and Dylan eil Ton, as well as the earliest mention in any western vernacular of the feats of Hercules and Alexander. The manuscript is incomplete, having lost a number of its original leaves, including the first.”
He is associated with Arthur, in part because he wrote so much about him, but that he was a court poet dates to the 11th century Welsh work, Culhwch and Olwen.
Scholars are divided as to how many poems are attributable to Taliesin. Of the 57 poems in the Red Book, those that are addressed to rulers of Wales at the time are confirmed as his. The rest are on mythological and religious topics. Some scholars imply that these are thus of a later date, and that wouldn’t be unusual, in that it was not uncommon when transcribing a book to attribute later works to the original source.
The open-source translations of Taliesin’s poetry are not necessarly the best, most poetic, or most accurate, but here is the source: http://www.maryjones.us/ctexts/llyfrtaliesin.html
Taliesin as myth is another person entirely. Within the Welsh mythology, and then later the Arthurian legend, Taliesin becomes a prophet-bard. A good summary of the mythology is found here: http://www.timelessmyths.com/celtic/mabinogion.html#Taliesin