Morgana, or Morgaine le fay, was half-sister to King Arthur. She was a powerful enchantress, scarcely inferior to the Lady of the Lake herself, but otherwise a lady of a very indifferent character, "born of fire and lasciviousness," as Merlin more correctly than courteously describes her.
However, it must be stated in mitigation of her offence, that in laying a trap for the virtue of her sister-in-law, she was but repaying Queen Guinevere in kind. For we learn from the Table-Round Romances, that on a former occasion the Queen had maliciously conducted Arthur to Morgana's bower, at a critical conjuncture in a love-tryste, similar to that into which she here inveigles Launcelot and Guinevere.
SIR GAWAIN was nephew to King Arthur, by his sister Morgana, married to Lot, king of Orkney, who was by Arthur made king of Norway. Sir Gawain was one of the most famous knights of the Round Table, and is characterized by the romancers as the sage and courteous Gawain. To this Chaucer alludes in his “Squiere’s Tale,” which the strange knight “saluteth” all the court–
“With so high reverence and observance,
As well in speeche as in countenance,
That Gawain, with his olde curtesie,
Though he were come agen out of faerie,
Ne coude him not amenden with a word.”
Gawain’s brothers were Agravain, Gaharet, and Gareth.
[See: The Squire's Tale From The Canterbury Tales by Geoffrey Chaucer]
[See: Sir Gawayne and The Green Knight An Alliterative Romance Poem]
Once upon a time King Arthur held his court in merry Carlisle, when a damsel came before him and craved a boon. It was for vengeance upon a caitiff knight, who had made her lover captive and despoiled her of her lands. King Arthur commanded to bring him his sword, Excalibar, and to saddle his steed, and rode forth without delay to right the lady’s wrong. Ere long he reached the castle of the grim baron, and challenged him to the conflict. But the castle stood on magic ground, and the spell was such that no knight could tread thereon but straight his courage fell and his strength decayed. King Arthur felt the charm, and before a blow was struck his sturdy limbs lost their strength, and his head grew faint. He was fain to yield himself prisoner to the churlish knight, who refused to release him except upon condition that he should return at the end of a year, and bring a true answer to the question, “What thing is it which women most desire?” or in default thereof surrender himself and his lands. King Arthur accepted the terms, and gave his oath to return at the time appointed. During the year the king rode east, and he rode west, and inquired of all whom he met what thing it is which all women most desire. Some told him riches; some pomp and state; some mirth; some flattery; and some a gallant knight. But in the diversity of answers he could find no sure dependence. The year was well nigh spent when, one day, as he rode thoughtfully through a forest, he saw sitting beneath a tree a lady of such hideous aspect that he turned away his eyes, and when she greeted him in seemly sort made no answer. “What wight art thou,” the lady said, “that will not speak to me? It may chance that I may resolve thy doubts, though I be not fair of aspect.” “If thou wilt do so,” said King Arthur, “choose what reward thou wilt, thou grim lady, and it shall be given thee.” “Swear me this upon thy faith,” she said, and Arthur swore it. Then the lady told him the secret, and demanded her reward, which was that the king should find some fair and courtly knight to be her husband.
King Arthur hastened to the grim baron’s castle and told him one by one all the answers which he had received from his various advisers, except the last, and not one was admitted as the true one. “Now yield thee, Arthur,” the giant said, “for thou hast not paid thy ransom, and thou and thy lands are forfeited to me.” Then King Arthur said:–
“Yet hold thy hand, thou proud baron,
I pray thee hold thy hand.
And give me leave to speak once more,
In rescue of my land.
This morn, as I came over a moor,
I saw a lady set,
Between an oak and a green holly,
All clad in red scarlet.
She says all women would have their will,
This is their chief desire;
Now yield, as thou art a baron true,
That I have paid my hire.”
“It was my sister that told thee this,” the churlish baron exclaimed. “Vengeance light on her! I will some time or other do her as ill a turn.”
King Arthur rode homeward, but not light of heart; for he remembered the promise he was under to the loathly lady to give her one of his young and gallant knights for a husband. He told his grief to Sir Gawain, his nephew, and he replied, “Be not sad, my lord, for I will marry the loathly lady.” King Arthur replied:–
“Now nay, now nay, good Sir Gawaine,
My sister’s son ye be;
The loathly lady’s all too grim,
And all too foule for thee.”
But Gawain persisted, and the king at last, with sorrow of heart, consented that Gawain should be his ransom. So, one day, the king and his knights rode to the forest, met the loathly lady, and brought her to the court. Sir Gawain stood the scoffs and jeers of his companions as he best might, and the marriage was solemnized, but not with the usual festivities, Chaucer tells us:–
“There was no joye, ne feste at alle; N’as is not was, contracted; in modern phrase, there was not. Mockel sorwe is much sorrow: morwe is morrow.
There n’as but hevinesse and mochel sorwe,
For prively he wed her on the morwe,
And all day after hid him as an owle,
So wo was him his wife loked so foule!” 
When night came, and they were alone together, Sir Gawain could not conceal his aversion; and the lady asked him why he sighed so heavily, and turned away his face. He candidly confessed it was on account of three things, her age, her ugliness, and her low degree. The lady, not at all offended, replied with excellent arguments to all his objections. She showed him that with age is discretion, with ugliness security from rivals, and that all true gentility depends, not upon the accident of birth, but upon the character of the individual.
Sir Gawain made no reply; but, turning his eyes on his bride, what was his amazement to perceive that she wore no longer the unseemly aspect that had so distressed him. She then told him that the form she had worn was not her true form, but a disguise imposed upon her by a wicked enchanter, and that she was condemned to wear it until two things should happen; one, that she should obtain some young and gallant knight to be her husband. This having been done, one half of the charm was removed. She was now at liberty to wear her true form for half the time, and she bade him choose whether he would have her fair by day and ugly by night, or the reverse. Sir Gawain would fain have had her look, her best by night, when he alone should see her, and show her repulsive visage, if at all, to others. But she reminded him how much more pleasant it would be to her to wear her best looks in the throng of knights and ladies by day. Sir Gawain yielded, and gave up his will to hers. This alone was wanting to dissolve the charm. The lovely lady now with joy assured him that she should change no more; but as she now was so would she remain by night as well as by day.
“Sweet blushes stayned her rud-red cheek,
Her eyen were black as sloe,
The ripening cherrye swelled her lippe,
And all her neck was snow.
Sir Gawain kist that ladye faire
Lying upon the sheete,
And swore, as he was a true knight,
The spice was never so swete.”
The dissolution of the charm which had held the lady also released her brother, the “grim baron,” for he too had been implicated in it. He ceased to be a churlish oppressor, and became a gallant and generous knight as any at Arthur’s court.