Talisein was a seer to the great kings of old. He died around 300 years before the events of The Crystal Cave. though an apparition of Taliesin appeared to Merlin 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 the apparition of Talisein appeared- 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/has the power to return from the dead at his own will.