This does not appear to be the same character as Arthur’s nephew, because he is named as a king, because he is given a brother named Segures, and because no mention is made of the relationship between Mordred and Guinglain. On the other hand, Segures is similar to Seguarades, who is Mordred’s foster-brother in the Post-Vulgate Merlin continuation.
Medraut, Medrawd, Medrawt, Medrod, Modreuant, Modred, Morddrede, Modret, Mordet, Mordarette, Mordered, Mordrech, Mordrés, Mordret
At Carlion, shortly after the battle of Bedegraine and before his marriage with Guenevere, Arthur engendered Mordred in conscious adultery and unconscious incest upon his visiting half-sister, King Lot's wife Margawse. That same night, Arthur dreamed of a serpent which came forth from his side, destroyed his land and people, and fought with him to their mutual destruction. The nightmare was so vivid that Arthur had it pictured in a painting in Camelot cathedral.
Some little time later, probaby shortly before or shortly after Mordred's birth in Orkney, Merlin told Arthur that the child who would destroy him would be born on May Day, thus inciting Arthur to send for all noblemen's sons born about that time, put them into a leaky ship, and send them out to sea. The infant Mordred was among these children, but when the ship went down, he was cast up on shore, where
a good man found him, and nourished him till he was fourteen years old, and then he brught him to the court ... toward the end of the Death of Arthur.
Other evidence, however, suggests that the boy was somehow identified and returned to his mother to be raised and educated by her; certainly he was known to be the youngest brother of Gawaine, Agravaine, Gaheris, and Gareth. Possibly he was identified when the "good man" brought him to court at age fourteen and returned to Margawse at that time for five or six years. Meanwhile, Arthur's Herod-trick had inspired Lot and other nobles to fresh revolt, in which Lot was killed.
Vulgate IV tells us that Mordred was knighted at the age of twenty. He was tall with fair curly hair, and would have been handsome but for a wicked expression. Only for the first two years of his knighthood did he do any good. He hated all good knights. During those first two years, however, he seems to have been very promising. For a time he traveled adventuring with Lancelot. Together they saw the mystic stag and four lions in Carteloise Forest, and Mordred won praise from the great Du Lac for his manly endavors. All this time, Mordred believed himself the son of King Lot.
Unfortunately, after seeing the stag and lions, Lancelot and Mordred went on toward Peningues Castle to attend a tournament. They stayed with a vavasour near the castle and went into the woods next morning to find a church or chapel at which to hear Mass.
They came upon an old but vigorous priest praying at a magnificent tomb. This priest greeted Lancelot and Mordred as the two most unfortunate knights who ever lived. When they asked why, he began with Mordred. First he stripped away Mordred's belief about his parentage: Mordred was not Lot's son, but Arthur's, the serpent of Arthur's dream, who whould destroy his father and do more harm in his lifetime than all his ancestors had done good, and so on, and so on.
It was rather extreme of Mordred to kill the priest, but then, all this must have been a very traumatic revelation for a young knight (no older than twenty-two) who had been winning praise until that morning. Other knights, like Lionel and Lancelot himself, did as much and more, often on less provocation, when the battle rage took them. The vavasour was greatly perturbed at the priest's death, but for Lancelot's sake said nothing. Lancelot would have found a pretext to kill Mordred, but refrained for the sake of Mordred's brother Gawaine.
(Lancelot was in a shaky position to cast stones as Mordred - among the victims of Lancelot's battle rages were men whose only offense consisted in Lancelot's having made free with their pavilions wihout first apprising them of his presence - but this time Lancelot was annoyed because Mordred had killed the priest before he could get around to predicting Lancelot's own future. Lancelot should have been grateful for that.)
They went on to hear Mass and then to the tournament. Such was the temper of the times. That day Mordred was left in a pitiable state on the tournament field, for he would have preferred death to surrender.
The old priest's prophecy seems to me one of the most mischievous in all the cycle (tying only with Merlin's about the May baby). It formed the turning point of Mordred's career, for after this episode his evil side took the upper hand. Lancelot, on returning to court, told Guenevere of the prophecy, but did not add that Arthur was Mordred's father. Guenevere did not believe the prophecy, and so did not mention it to Arthur, who might have banished Mordred had he known of the episode.
Whether on the strength of his early promise, or because of his family connections, or because he remained a competent fighter "of his hands", Mordred became a companion of the Round Table. He seems to have retained some sense of humor; once he joined Sir Dinadan and others in playing a joke on a "Cornish knight" (King Mark): Mordred's shield was silver, with black bends. Dinadan told Mark it was Lancelot carrying this shield, after which Mordred, who was wounded, gave the shield and his armor to Dagonet, the jester, who then gave Mark a merry chase. ([This same shield suggests that Mordred may have been allied, at least for a time, with Morgan le Fay.) Again, one time Mordred came upon Sir Alisander le Orphelin in a state of besottedness upon his lady love, and began leading him mockingly away, apparently for mere sport. When Percivale came to court, Mordred apparently joined Kay in mocking the young man.
More serious, he may well have been party to the scheme which culminated in Gaheris' murder of Margawse, and when the brothers tracked down Lamorak, it was Mordred who gave that knight his death wound, striking him from behind. Mordred and Agravaine also conceived a dislike for Dinadan because of the latter's friendship toward Lamorak, and during the Grail Adventures they found an opportunity to kill him.
At last Mordred and Agravaine conspired to corner Lancelot with the Queen. Mordred suvived Lancelot's escape and must have played chief witness against the lovers, thus precipitating the break between Arthur and Lancelot. When Arthur went with Gawaine to attack Lancelot in France, he left Mordred as regent and "chief ruler of all England", with governance even over Guenevere. Malory says Arthur did this because Mordred was his son, but this would seem to make Arthur surprisingly slow-witted about connected Merlin's old prophecy and his own nightmare with Mordred, so it is my guess that Arthur may still have been unaware of the relatioship and made Mordred his regent because Mordred was the last surviving brother of the King's favorite nephew Gawaine.
Left in charge, Mordred counterfeited letters telling of Arthur's death in battle. He then called a parliament to name him king, had himself crowned at Canterbury, and tried to marry Guenevere, but she tricked him and barricaded herself in the Tower of London. Mordred drove the Archbishop of Canterbury into exile for opposing him and besieged Guenevere, but withdrew on receiving word that Arthur was on his way back. Mordred tried to prevent Arthur's landing at Dover and retreated to Canterbury. The last battle was fought on Salisbury Plain (Camlann?); Mordred was the last man left alive of all his army and allies who fought there, and at the last he was killed by Arthur - though Mordred did not return Arthur a mortal blow until he felt that he himself had his death wound.
Among Mordred's allies were the Saxons or Sesnes, who hated Arthur and wanted revenge on him. Mordred also had Irish, Scottish, and Welsh divisions in his army. He left behind two grown sons, one named Melehan and one whose name is not given. They gradually seized England after Mordred's death, but, on Lancelot's return, Bors killed Melehan and Lancelot the other son.
Medraut and Medrawt is the Welsh version of Mordred in various sources, such as Annales Cambriae.
Sir Mordred's Family and Allies