The Barddas of Iolo Morganwg, Vol. I., ed. by J. Williams Ab Ithel, , at sacred-texts.com
It was in the time of Owain, son of Maxen Wledig, that the nation of the Cymry recovered their privilege and crown. They took to their primitive mother tongue instead of the Latin, which had well nigh overran the Isle of Britain; and in the Cymraeg they kept the memorials and history and systems of country and nation, restoring to memory the ancient Cymraeg, with its original words and expressions. Because the ancient orthography of the ten primary letters was forgotten and misunderstood, they became lost, and thus arose a disagreement respecting several old words, that is, the putting of two letters, where only one was required, as caan, braan, glaan, instead of cân, brân, and glân, and digerth instead of dierth, and phlegid instead of phlaid, with many others; also putting t for dd, and i instead of e, and instead of y, and u instead of e. It is not necessary to show the whole, but this much is given in memory of him who made the amendment, namely, Talhaiarn
p. 62 p. 63
the Bard, 1 of Caerleon-upon-Usk, under the protection of the Round Table. After him Taliesin, Chief of Bards, arranged the Cymraeg, from a right understanding of the meaning and merit of the ten primary letters, and their modes, and changes, and proper inflections; and from this the ancient Cymraeg was restored and recovered.
63:1 p. 62 Talhaiarn presided in the chair of Urien Rheged, which was established at Caer Gwyroswydd, or Ystum Llwynarth. He composed a prayer, which has always been the formula used in the Gorsedd Morganwg, or Bardic Sessions of Glamorgan. He was also domestic chaplain to Emrys Wledig, or Ambrosius Aurelianus. Taliesin in his Poems alludes to Talhaiarn,--
According to the language of Talhaiarn,
There will be baptism at the day of judgment.