Bk XII:146-209 Nestor tells the story of Caeneus-Caenis.
This battle brought about that truce, of many days duration, when both sides grounded their weapons and rested. While alert sentries patrolled the Trojan walls and alert sentries patrolled the Greek trenches, a feast day arrived, on which Achilles, the victor over Cycnus, was propitiating Pallas with the blood of a sacrificial cow. When its entrails had been placed on the blazing altars, and the perfume the gods love had climbed to the heavens, part was put aside for their holy rites, and the rest set out on the tables. The leaders reclined on couches, and ate their fill of the roasted meat, while they quenched their thirst, and drowned their cares, with wine. The zither, the sound of singing, the long boxwood flute pierced with many holes, was not their entertainment, rather they lengthened the night with talk, and courage was their theme. They talked of their enemies’ battles, and of their own, and delighted in recounting, in turn, the dangers they had encountered and survived. What else should Achilles speak of, and what else should be spoken of in great Achilles’s presence?
The foremost talk was of his latest victory, the overthrow of Cycnus. It seemed wondrous to all of them that a warrior should have a body no spear could penetrate, impervious to wounds, and that blunted iron swords. Achilles himself and the Greeks were marvelling at it, when Nestor said: "Cycnus has been the only one among your generation who ignored swords, and whom no blow could pierce. But, long ago, I myself saw one Caeneus of Thessaly, who could take a thousand strokes with unwounded body: Thessalian Caeneus, I say, who, famous for his exploits, lived on Mount Othrys, and what made it more remarkable in him, he had been born a woman." All who there were interested by this strange wonder, and asked him to tell the story.
Achilles, among the rest, said: "Say on, old one! O ancient eloquence, wisdom of our age, all of us equally desire to hear, who Caeneus was, why he was changed to his opposite, what campaign you met him in, fighting against whom, by whom he was overcome, if anyone overcame him." Then the old warrior said: "Though the slowness of age hampers me, and many things I once saw have slipped from me, I can still remember many. Nothing sticks more firmly in my mind than this, amongst all those acts, in battle and at home, and if length of years alone enabled a man to report many deeds, I have lived two hundred years: now I live in my third century.
Elatus’s daughter, Caenis, loveliest of the virgins of Thessaly, was famous for her beauty, a girl longed for in vain, the object of many suitors throughout the neighbouring cities and your own (since she was one of your people, Achilles). Perhaps Peleus also would have tried to wed her, but he had already taken your mother in marriage, or she was promised to your father. Caenis would not agree to any marriage, but (so rumour has it) she was walking along a lonely beach, and the god took her by force. When Neptune had enjoyed his new love he said: 'Make your wish, without fear of refusal. Ask for what you most want!' (The same rumour mentioned this.)
'This injury evokes the great desire never to be able to suffer any such again. Grant I might not be a woman: you will have given me everything,' Caenis said. She spoke the last words in a deeper tone that might have been the sound of a man’s voice. So it was: the god of the deep ocean had already accepted her wish, and had granted, over and above it, that as a man Caeneus would be protected from all wounds, and never fall to the sword. Caeneus, the Atracides, left, happy with his gifts, and spent his time in manly pastimes, roaming the Thessalian fields."