When he was a teenager, he fell into a trance one day while skating and saw that the son of Gwyddno Garanhir had been thrown into a dungeon.  
In one of his writings this man claims to be the patriarch of the prophet Elijah and to have seen the destruction of Sodom and Gomorrah.  When he was a teenager, he fell into a trance one day while skating and saw that the son of Gwyddno Garanhir had been thrown into a dungeon.  This man then tricked the repulsive Rhun into cutting off a servant's finger, thereby protecting the reputation of Elphin's wife.  Later, he defeated Rhun's father, the unpleasant king Maelgwn (mail-gwin), in a horse race.  As a child, he transformed himself into a grain of wheat to escape Ceridwen, but she turned into a hen and swallowed him.  FTP, name this magician and bard of Welsh legend, whose name was originally Gwion and who was much admired by Frank Lloyd Wright.
In one of his writings this man claims to be the patriarch of the prophet Elijah and to have seen the destruction of Sodom and Gomorrah.  
Taliesin (accept Gwion before it is mentioned)