A Knight of the Round Table who became a hermit.
According to the Vulgate Merlin, he was the son of Bron, the first Fisher King, and Joseph of Arimathea's sister, Havingues (Enygeus), which means, to be living in Arthurís time, that he had an extremely long life span. His brothers included Alan and Joshua. The Livre díArtus more reasonably makes him a descendant of the first Nascien.
He traded his armor for a robe after he witnessed the Grail carried by a white stag. He retired to a hermitage and became a sage for Arthurís knights during the Grail Quest. He may have tutored Galahad. He was visited by Gawain and Hector, and he told them both that they would not achieve the Grail because of their sins.
Some commentators make the assumption that he is the same as the second Nascien above, but this is unfounded.
In Welsh legend, the son of the King of Denmark, and one of Arthurís Three Royal Knights. He is described as wise, handsome, and skilled at arms. Likely, Welsh storytellers simply borrowed the name from the Grail stories.
Nasiens | The Legend of King Arthur
Naciamis, Nacien, Naciens, Nacyanus, Nancien, Nanciens, Nasciens, Nasiens, Nassien, Nassijen, Natianus, Natiien, Necienzo
Seraphe was the brother-in-law of King Evelake. Joseph of Arimathea converted them and their families to Christianity, giving Seraphe the baptismal name of Nascien and Evelake that of Mordrains. Nascien's wife was Flegentine, their son Celiodine, and among their descendants were the Fisher Kings, Lancelot and Galahad, and Percivale and Amide.
Mordrains, Nascien, and their families came to Britain with Joseph and Josephe. Here, forty years after Christ's Passion, Nascien successfully endured temptation by a fiend in the Port of Perilous Rock. Mordrains, meanwhile, came to the Isle of Turnance, found King Solomon's Ship, and broke King David's Sword by drawing it unworthily to slay a giant that was chasing him. Somehow killing the giant without the use of the sacred weapon, Mordrains returned to Solomon's Ship and was carried to the Port of Perilous Rock, where Nascien, fresh from his testing, miraculously repaired the sword and returned it to its sheath. They were warned by a voice to leave Solomon's Ship, and, in so doing, Mordrains recieved a sword wound in the right foot as punishment for drawing the sword. (In the passage Malory XVII, 4, Nascien is credited with drawing King David's Sword and Mordrains with its mending, but, convinced that the two names have been transposed here, I have recapped the tale accordingly.)
Surviving as a holy hermit until the reign of Arthur (or perhaps returning as an apparition), Nascien filled very approximately the role in the spiritual world of the Grail Adventures that Merlin had formerly played to Arthur and his knights in the secular world: Nascien was their more or less elusive prophet, mentor, guide, and confessor. I believe he is to be identified with Malory's unnamed hermit who came to the Round Table the Whitsunday before Galahad's birth and predicted that event within the year, and with the "good old man, and ... ancient, clothed all in white, and there was no knight knew from whence he came" who actually brought Galahad into Arthur's court.
As Galahad succeeded in drawing Balin's sword from its floating rock, a damsel riding a white palfrey appeared on the river bank to tell Lancelot he was no longer the greatest knight of the world and to bring Arthur Nascien's message "that thee shall befall the greatest worship that ever befell king in Britain". Just before the departure of the Questers, "an old knight ... and in religious clothing" came into court to give them Nascien's warning and instructions for the holy enterprise, among which was the injunction that no knight should be allowed to take along a woman. (Interesting, and also in view of Nascien's own use of a damsel as one of his messengers.) When Galahad came to the Abbey of the Adventurous Shield, the angelic White Knight, telling the story of the shield, mentioned Nascien's death and burial in the same abbey.
Here is one in a little mountain, but it is so rough there may no horse go thither, and therefore ye must go upon foot; there shall ye find a poor house, and there is Nascien the hermit, which is the holiest man in this country.
Then [they] rode till that they came to the rough mountain, and there they tied their horses and went on foot to the hermitage. And when they were come up they saw a poor house, and beside the chapel a little courtelage [courtyard], where Nacien the hermit gathered worts, as he which had tasted none other meat of a great while.
He explained their dreams and, finding them unworthy, gave them some harsh counsel, which they rejected. It is not unlikely that Nascien should also be identified with some of the nameless holy men who counseled other knights on this Quest, nor does it seem unlikely that, if he had indeed spent several centuries of miraculously prolonged life in Britain, he might have had more than one hermitage scattered about the island.
According to Vulgate VII, Nascien's mother was la bele Damoisele de la Blanche Nue (or Nuage) - the lovely maiden of the white cloud. I am not completely sure, however, that this is the same Nascien. Here he appears in the thick of battle, helping sixteen other knights succor Agloval. I remember no other testimony that the hermit Nascien indulged in marital activities during Arthur's reign.