Taliesin

"He died, I'd say, about 300 years ago. Taliesin was a seer to the great kings of old. It is said that the Crystal Cave was a source of his prophecies."

- Gaius on Taliesin

Taliesin was a seer to the great Kings, in the time of Old Religion. He apparently died around 300 years before he met Merlin, though he appeared and showed him to the Crystal Cave. It is to this cave that he attributed his power of prophecy, as he was aware of Merlin's destiny and seemed to have foreseen their meeting. It is unknown exactly how was the apparition of Taliesin - he may have been a ghost or an illusion created by the cave, though he was shown to be capable of using magic, and so perhaps he never died at all. It is also possible that he had the power to return from the dead at his own will.

Biography
When Merlin was sobbing over the possibility of Arthur dying, Taliesin appeared to him and asked him "why are you crying?" Merlin then informed him of Arthur's seemingly fatal wound. A confident Taliesin proclaimed that Arthur's "time for him to die is not yet upon us". Taliesin then approached Arthur, and casted a spell on him which healed his wounds. Then Taliesin told Merlin he needed to show him something. Taliesin took him to a nearby cave filled with magical crystals - he identified it being called 'the Crystal Cave'. Merlin then walked inside, he immediately saw images that did not please him at all, he then vowed not to look in them as the last time he looked into a magical crystal, he saw terrible things. However Taliesin insisted that he looked into them, saying "use what you see, for good!" Merlin then reluctantly turned his head and did so, he then again saw terrible things, and by the the time he looked away, the Taliesin had mysteriously vanished (The Crystal Cave).

Abilities
Taliesin appeared to have possessed various magical abilities and knowledge since he seemed to be dwelling around the Crystal Cave for hundreds of years. He was seen using a healing spell in which Merlin could not cast which resulted in him healing the wounded Arthur. Taliesin's eyes turned orange when casting the healing spell as oppose to the original sorcerer's yellow.

In the Legend
Taliesin was born in 534 and died in 599. He was the bard of Urien, king of Reghed, and then of his son, Owain. We can read twelve of his poems, in the Llyfr Taliesin, The Book of Taliesin (eight were written for the king Urien).