Hen Gedymdeith, originally known as "Gerhart of Aelle", was the oldest living mage before the Thanedd coup. A member of the Brotherhood of Sorcerers, he was also the author of Elves and Humans. In fact, he was the child of two of the first human settlers on the continent which would make him nearly 500 years old at the time of the Thanedd coup. He did not survive the coup as his heart gave out.
|“|| The assembled sorcerers parted, bowing with respect at the personages entering the hall. The first was a middle aged but vigorous man in extremely modest woollen clothing. At his side strode a tall, sharp-featured woman with dark, smoothly combed hair.
"That is Gerhart of Aelle, also known as Hen Gedymdeith, the oldest living sorcerer," Yennefer informed Geralt in hushed tones.
— pg(s). 117, Time of Contempt (UK edition)
|“|| Marti Södergren, the healer, was sitting in the window alcove. Hearing his footsteps, she raised her head. Her cheeks were wet with tears.
"Hen Gedymdeith is dead," she said in a faltering voice. It was his heart. I couldn't do a thing ... Why did they call me so late? Sabrina hit me. She hit me in the face. Why? What has happened?"
— pg(s). 174, Time of Contempt (UK edition)
According to this compendium of in-universe information, Hen Gedymdeith was author of The Legends of the Elder Races where he described tales about Aen Elle.