The Death of Arthur and Mordred

Mordred is Arthur's nephew; no Lancelot; 
battle at Tamar/Camblan
Mordred is Arthur's son; Lancelot story; 
battle at Salisbury Plain
Geoffrey of Monmouth Mort Artu (Vulgate Lancelot-Grail)
Wace Post-Vulgate Lancelot
Layamon Stanzaic Morte
Alliterative Morte Malory's Morte Arthur

First, three suggestions in pre-Geoffrey materials that Arthur had a quarrel or relationship with a man whose name began with M, who might have taken his wife captive:

1. Annals of Cambria

539 AD: The Battle of Camlann, in which Arthur and Medraut both fell; and there was a widespread death in Britain and Ireland.

2. Modena Archivolt (sculpture):

Summary: Artus de Bretania, Isdernus, Galvagin, Galvariun, and Che are shown galloping to rescue Winlogee from a tower in which she is held by Mardoc and Carrado.

3. Life of Gildas by Caradoc:

Summary: Melwas, king of Somerset, abducts Arthur's wife Guennuar and holds her in Glastonbury. Arthur comes with troops to rescue her but is deterred by the marshy terrain; St. Gildas intervenes and restores his wife to Arthur.

Geoffrey of Monmouth, History of the Kings of Britain (trans. Thorpe, Penguin):
[Arthur, conquering the Roman empire on the Continent, hears that his nephew Mordred, to whom he entrusted Britain in his absence, has crowned himself and is living in sin with Guenevere. Arthur of course returns and engages Mordred the Perjuror in several battles, the last of them at the River Camblann.]
Mordred was indeed the boldest of men and always the first to launch an attack....
.... Arthur, with a single division in which he had posted 6,666 men, charged at the squadron where he knew Mordred was. They hacked a way through with their swords and Arthur continued ot advance, inflicting terrible slaughter as he went. It was at this point that the accursed traitor was killed and many thousands of his men with him. However, the others did not take to flight simply because Mordred was dead.... Arthur himself, our renowned King, was mortally wounded and was carried off to the Isle of Avalon, so that his wounds might be attended to. He handed the crown of Britain over to his son Constantine, the son of Cador Duke of Cornwall; this in the year 542 after our Lord's Incarnation.

Wace (The Life of King Arthur, Wace and Lawman, Everyman; trans. Weiss):
[Arthur, campaigning in Burgundy against the Roman emperor, hears that Modret, his sister's son, left in charge of Britain, is receiving homage and hostages from the lords and has taken the king's wife to his own bed. He returns and chases Modret across Britain until Modret makes a last stand in Cornwall.]
The battle was beside Camble (Cambre) in the land of Cornwall.... Modret was slain in the fray, and  the vast majority of his people besides, and the cream of Arthur's people, both the strongest and the best. Arthur, if the chronicle is true, received a mortal wound to his body. He had himself carried to Avalon for the treatment of his wounds. He is still there, awaited by the Britons.... Truly, 542 years after the Incarnation he did have himself carried to Avalon. It was a great loss that he had not children. To Cador's son, Costentin of Cornwall, his cousin, he surrendered his kingdom, and told him to be king until he returned.

Layamon (The Life of King Arthur, Wace and Lawman, Everyman; trans. Rosamund Allen):
[Same action as Wace, but with many more speeches.]
[At Camelford] Upon the River Tamar they encountered each other....
Modred was there slain and deprived of his life-days,
And all of his knights were slain in the fight;
There were slain al the sprightly courtiers of Arthur...
And Arthur was badly wounded with a broad halberd;
Fifteen appalling wounds he had on him;
Into the very least of them two gloves could be thrust!
Then there were no more who survived the battle...
Save King Arthur alone, and of his knights just two.
[Arthur is approached by a young lad of his clan, Constantine, and bequeaths him the kingdom, saying he himself is going to be healed in Avalon by Queen Argante, and will return.]

Alliterative Morte Arthure (King Arthur's Death, trans. Brian Stone)
[Arthur leaves Guenevere and his "cousin" Mordred as co-protectors of Britain, and Mordred as Guenevere protector, while he goes off to conquer the Roman Empire. Mordred seizes the crown, revenues, and Guenevere, who hs borne him children by the time Arthur hears of it. Arthur returns and meets Mordred the Malebranche where the latter is encamped by the River Tamar in Cornwall. Mordred is wielding Arthur's father's sword Clarent (which Guenevere gave him), but Arthur has Excalibur.]
Our Prince perceived it, and pressed fast forward,
Hurtling through he host with his whole strength.
There he met Mordred and with full malice said,
 "Turn, untrue traitor, your time is up!
But the great God I shall give you your death-blow.,
And no rescue or ransom shall reach you from any man!"
The sovereign struck him staunchly with Excalibur,
Shearing off the corner of the shining shield
And hitting a hand's-breadth deep into the shoulder,
So that the bright red blood blazoned the mail.
Mordred shuddered and shivered, but shrank back little,
Rather shot forward sharply in his shining gear,
And the felon struck fiercely with that fine sword,
Ripping through the rib-plates on Arthur's right side.
Through surcoat and hauberk of armoured steel
the hilding hacked off a half-foot of flesh.
That deadly blow brought his death, and dread pity it was
That the dauntless man should die but by God's deeming!
Yet still with his sword Excalibur he struck nobly,
Guarding himself guilefully with his glittering shield,
And slashed off Mordred's sword hand as he surged past.
An inch from the elbow he hacked it clean off,
So that Mordred sank down and swooned in the dust;
Yes, through brassard of bright steel and brilliant mail,
And hilt and hand upon the heath were left lying.
Then deftly he dragged that devil upright again
And broached him with the blade to the bright hilts,
So that he squirmed on the sword-point in his death-struggle.
"In faith, said the fated king, "It fills me with grief
That such a false felon should have so fair a death."
[Arthur 's men now rout Mordred's but as Arthur begins to mourn the bodies of his slain friends he feels ready to die. He and his men go to Glastonbury and enter the Isle of Avalon, where a doctor from Salerno tries to cure Arthur in vain. He bequeaths his crown to  his cousin Constantine, asks that Mordred's children be slain but Guenevere spared, and dies.]

La Mort le roi Artu (ed. Frappier, trans. J. Shoaf)
[Mordret, the king's son and sister's son, is left in charge of Britain when Artu leaves to fight Lancelot in France. Guinevere is returned to Artu, and we are told that she is left in Mordret's charge. Artu while on the continent defeats some Romans in Burgundy and is considering marching on Rome. Mordret forges a letter saying Arthur is dying and bequeaths the queen (and presumably the kingdom) to Mordret, but she shuts herself up in the Tower of London and sends a message to Artu, who swears he will kill his son with his own two hands. Artu returns and a great battle takes place on Salisbury Plain. Arthur ignores a warning dream. Late in the battle, Mordret the Perjured commands an attack on the king's standard.]
    ...and King Artu, who recognized Mordret easily, faced him, and Mordret did too; they struck each other like brave, strong knights. Mordret struck the King first, piercing his shield, but the hauberk was so strong that not a link of the mail broke, and the sword shattered with the stroke, and the King did not even budge. And the king, who was strong and fierce and used to jousting, struck him with such power that he knocked him and his horse right down to the ground in a heap--but did no other damage, since Mordret was well-armed. [2,000 of Mordret's men rush to protect and remount their lord.]
    But Mordret was a good brave knight; he charged at Artu to get revenge, for he was unhappy over having been upset while among his own forces; and the King did not refuse to fight, but pulled around his horse's head to meet him, and they struck each other with such great blows of edged spears that they stunned each other and could scarcely stay in their saddles, and kept themselves from falling to the earth only by holding on to their horses' tails. [Now Mordret personally slays Yvain, and the battle gets fiercer until only 4 of the Round Table knights are left; Mordret charges and kills one of them, Sagramor, who was already severely wounded.]
    When the King saw this blow, he lamented, "Oh, God, why have you let me drop so low in worldly power? For love of this blow, I vow to God that either Mordret or I will die here." He held a great strong sword, and charged with as much power as he could get from his horse; and Mordret, who recognized the King and wanted nothing more than to slay him, did not refuse to fight, but turned his horse's head to meet him, and the King, who was coming at him with all his strength, struck him so hard that broke the mail of the hauberk and ran his sword blade right through him. It is said that after the sword was pulled out, a ray of sunlight passed through the wound so clearly that Girflet saw it, and the folk said that it was a sign of God's wrath. When Mordred realized how badly he was wounded, he knew it was to the death. He struck King Artu so hard on the helmet that it could not protect his head from the sword, and he cut off a piece of the head itself.
    King Artu was so stunned by this blow that he fell off his horse to the ground, and so did Mordret; thus both were in such straits that no-one was strong enough to lift either of them up, and they lay on the ground beside each other. Thus the father killed the son, and the son wounded the father mortally. [There follow the stories of Arthur accidentally crushing Lucan to death, Excalibur thrown by Girflet back into the water to the arm that rises up, the boat of ladies, and Arthur's tomb found by Girflet.]

Post-Vulgate Death of Arthur (Lancelot-Grail, ed. Lacy, vol.  V; trans. Martha Asher)
[Summarizes Mordred's plot as in the Mort Artu. Arthur defeats the Romans not in Burgundy but in Brittany.]
    This was the treason Mordred committed against his uncle.... There was much good in Mordred, and as soon as he was elevated to the throne, he made himself well loved by all.
    [The two armies gather on Salisbury Plain.] In that battle Mordred fought and defended himself so well that everyone who saw him thought him a surprisingly good knight. And know that the story says that in his whole life he did not od as much in arms as on that one day, for with his own hands  he killed six of the companions of the Round Table.....
    King Arthur rode about the battlefield until he met Mordred, and he gave him such a great blow on the helmet that he laid him unconscious on the ground. He thought  Mordred was dead and said to him, "Mordred, much wrong you have done me, but it has brought you no profit."
    King Arthur knocked Mordred down, just as I have told you, but Mordred did not stay on the ground for long, for his vassals picked him up. When he was back on his horse, he felt deeply ashamed that he had fallen with his men watching. He charged at Sagremor and gave him such a blow that he knocked Sagremor's head far away....
    The king had already recovered his good, strong lance. He charged at Mordred, who was so courageous that he did not flinch, and struck him so hard that he put the lance right through his chest, and the wood appeared on the other side. And the story says that after he had pulled the lance back out, a ray of sunshine passed through the wound, so bright that Girflet saw it clearly, so that the people of the land, when they heard of it, said that it was a miracle of Our Lord and a sign of grief.
    Mordred knew that he was mortally wounded. He struck the king his uncle so hard that neither helmet nor iron coif kept the sword from penetrating to the bone and cutting off a large piece of his skull. With the blow, the king fell to the ground, and Mordred, too.   In this way, as I have told you, King Arthur killed Mordred, and Mordred wounded him mortally. And this was a great wrong, and a great loss, for there was not, after King Arthur, a Christian king so well favored....
    [Blioberis cried], "Now I see fulfilled the prophecy... that King Arthur would die by the hand of his son." [Arthur regained consciousness and said] "Mordred, in an evil hour did I beget you. You have ruined me and the kingdom of Logres, and you have died for it. Cursed be the hour in which you were born."
[To the subsequent account of the death of Lucan, the return of the sword to the lady of the lake, the boat of ladies, and the tomb, are added a couple of episodes: (1) Blioberis ties Mordred's body to his horse's tail and drags him until his body is torn to pieces and only the head is left. Arthur commands that the Tower of the Dead be built and Mordred's head hung there as a warning against treachery; this is done and the head remains until Charlemagne visits with Ganelon, who is so distressed by the head that he cuts it down and hides it. (2) Arthur the Less, Arthur's son, is killed by Blioberis, who sadly has his tomb engraved as the dying man requested.]

The Stanzaic Morte Arthur (King Arthur's Death, trans. Brian Stone)
[The plot is similar to that of the Mort Artu, except that there is no conflict with the Roman Empire.]

Sir Mordred, traitor false and vile,
King Arthur's sister's son,
(And Arthur's too, as all believe,
And that is why he won
The stewardship), was to the land
a wily, treacherous lord.
He wished to wed his father's wife,
Which many men abhorred.

[Guinevere locks herself in the Tower and is safe; the Archbishop of Canterbury curses Mordred. Arthur returns; warned by a dream, he hopes to avoid battle. But as the two parties approach a peace meeting at Salisbury, an adder bites one of Mordred's men who draws a sword to kill it. The battle begins, and continues until only Mordred, Arthur, Lucan, and Bedivere are left.]

"Shall we not fell this thief to earth?"
King Arthur fiercely raged,
And gripping his spear with grim intent
His foeman he engaged.

His spear hit Mordred in the chest
And through his backbone bore;
So Mordred lost his life at last
And never spoke word more.
But as he died, he raised his arm
And clove King Arthur's head
Through crest and helmet. Thrice he swooned
Like one that's almost dead.

[Lucan and Bedivere take Arthur to the chapel, where Lucan dies holding Arthur; Bedivere disposes of Excalibur to the Lady of the Lake, sees Arthur off in the boat of ladies to Avalon, and finds the tomb.]

Thomas Malory's Morte Arthur (Malory, Works, ed. Vinaver)
[The plot follows the Stanzaic Morte, with the forged report of Arthur's death, Guinevere locking herself in the tower, and the Archbishop's curse on Mordred's attempt at compound incest with "hys unclys wiff and hys fadirs wyff." At Salisbury Arthur is  moved to try for a truce by his dream, but Mordred's knight's sword drawn to kill an adder starts the battle, which continues until only the 4 men are left.]
    "Jesu mercy!" seyde the kynge, "where ar all my noble knyghtes becom? Alas, that ever I shulde se thys doleful day! For now," seyde kynge Arthur, "I am com to myne ende. But wolde to God," seyde he, "that I wyste now where were that traytoure sir Mordred that hath caused all thys myschyff."
[Lucan tries to discourage him from the encounter, but Arthur insists that he will never get another opportunity to kill Mordred.]
    Than the kynge gate his speare in bothe hys hondis, and ran towarde sir Mordred, crying and saying, "Traytoure, now ys thy dethe-day com!"
    And whan sir Mordred saw kynge Arthur he ran untyll him with hys swerde drawyn in hys hond, and there kyng Arthur smote sir Mordred undir the shylde, with a foyne of his speare, thorowoute the body more than a fadom. And whan sir Mordred felte that he had hys dethys wounde he threste hymselff with the myght that he had upp to the burre of kyng Arthurs speare, and ryght so he smote hys fadir, kynge Arthure, with hys swerde holdynge in both hys hondys, uppon the syde of the hede, that the swerde perced the helmet and the tay of the brayne. And therewith Mordred daysshed downe starked ded to the erthe.
    And noble kynge Arthure felle in a swoughe to the erthe, and there he sowned oftyntymes.....
[The rest of the elements are similar to the Stanzaic Morte.]