“Morgana” is the Latin form of the name of a “fay” or “sorceress'' who appears in many medieval Arthurian tales and in some non-Arthurian tales. Her name in French romances is usually “Morgain” or “Morgue”. This latter form might be the nominative form of the name and “Morgain” be the oblique form, but cases were breaking down in French at the time and they seem to be just treated as different forms of the same name, regardless of case. In Sir Thomas Malory’s English “Le Morte d’Arthur” she is called “Morgan” and this form of the name is often used by English translators for forms found in their texts.
In Geoffrey of Monmouth’s “Historia Regum Britanniae”, finished around 1136, which is the earliest surviving attempt to write a complete history of Arthur’s reign, Geoffrey ends with Arthur said to be taken away of the Island of Avalon to be healed of his wounds. Search on “isle of Avallon to be” at http://www.lib.rochester.edu/Camelot/geofhkb.htm .
In a later work by Geoffrey of Monmouth called the “Vita Merlini”, written about 1150, the island is known as the “Isle of Apples” and it is told by Taliesin that he brought the wounded Arthur there where Morgen dwelt with her eight sisters and she agreed to heal him. See http://www.sacred-texts.com/neu/eng/vm/vmeng.htm and search on “Morgen”. The eight sisters are mentioned in no other medieval text.
In what is arguably the earliest surviving French Arthurian romance, written perhaps about 1160, “Erec et Enide”, Chrétien twice brings in Morgain, once mentioning in passing that Guingamor, Lord of the Isle of Avalon was her “ami” and once having Erec healed by a plaster that Morgain had given to her brother Arthur. See http://www.gutenberg.org/etext/831 and search on “Morgan”. Morgain also appears as Arthur’s sister in most later texts, but not in all of them.
Somewhere between 1167 and 1170 the writer Etienne of Rouen wrote a work today known as “Draco Normanicus” in which he summarizes Geoffrey’s original account of Arthur’s passing to Avalon but also introduces Morgana as the ruler of Avalon and claims that Morgana was Arthur's sister.
Morgain is mentioned in some Arthurian tales as ruling in Sicily on Mount Aetna, known as Mount Gibel. It may be that Sicily, being a fertile island, was considered to be the real Avalon by some Bretons who lived among the Normans who possessed Sicily. The term “Fata Morgana” is still used of a mirage sometimes seen at the Straits of Messene. See http://en.wikipedia.org/wiki/Fata_Morgana_%28mirage%29 . Avalon is sometimes placed in the Mediterranean and not identified with Sicily.
In the “Story of Merlin”, attributed to Robert de Boron, the author makes out Morgain to be one of three daughters of the Duke of Tintagil whom he mentions, but makes clear that the Duke had other children as well. See http://kingarthur.wikia.com/wiki/Duke_of_Tintagel%E2%80%99s_Daughters . The name “Elaine” for one of them is only found in Malory. In the “Vulgate Merlin”, which lists five daughters, this one, the wife of King Nentres or King Neutres, is named Blasine.
As to Morgain and Guingamor: there is a romance of Guingamor in which he has a love affair with a fay and eventually leaves mortal lands to be with her. See http://www.archive.org/stream/guingamorlanvalt00mariuoft#page/xviii/mode/2up . A similar tale is told of a knight named Lanval and in this tale the king in the story is Arthur and the land of the fay is Avalon. See http://www.archive.org/stream/guingamorlanvalt00mariuoft#page/30/mode/2up . In both these tales, the Queen is the villain. The second continuation to Chrétien de Troyes' “Perceval” introduces a dead knight, named Brangamuer, the son of a certain Guingamuer and a fay named Brangepart.
In the “Prose Lancelot”, Morgain is said in her younger days to have had an affair with Guiomar, the nephew of Guenevere, until Guenevere came upon them and threatened to tell Arthur what his sister was doing unless it stopped at once. Guiomar, fearing what might happen, abandoned the pregnant Morgain, who then went off in search of Merlin whom she paid to teach her magic, which he did. Thereafter she lived as a powerful sorceress, was known by the “ignorant” as a goddess, and her son became a good knight. But she always hated Guenevere for breaking up her love affair and constantly attempted to avenge herself by telling Arthur about Guenevere’s affair with Lancelot. But events always conspired to seemingly disprove her accusations. Morgain‘s hatred of Guenevere is also referred to in the English poem “Sir Gawain and the Green Knight”.
It is suspected that this story derives from the old story of Guingamor, changed or perhaps accidentally garbled by the author or authors who sought to put Queen Guenevere more in the right and Morgain more in the wrong. (Malory brings in Sir Gringamore of Avalon (Avilion) in his story of Sir Gareth, but does not there connect him to Morgan the Fay. See http://www.sacred-texts.com/neu/mart/mart144.htm .)
In the “Prose Lancelot” Morgain tries to gain Lancelot’s love.
In prose romances later than the “Prose Lancelot”, Morgain is often hostile to Arthur and all his knights. When Tristan kills her lover Hunison (Hemison), she sends a magic spear to King Mark through which Tristan gets his death wound. In the “Post-Vulgate Arthurian Cycle” and in Malory she attempts to have Arthur slain in order that she may become queen and her lover Accolon may become king. But later in that cycle Arthur and Morgain seem to be rather friendly and Arthur sends to her for advice.
In the “Post-Vulgate Arthurian Cycle” (whence also in Malory) Morgain is the wife of King Urien and the mother of his son Yvain (Uwaine). That Yvain is son of Morgain is also found in the “Lay of Tyolet”. But earlier romances do not mention this. In the “Prose Lancelot”, Yvain appears with Morgain in circumstances that don’t suggest any kin relationship and Yvain seems not to be Arthur’s nephew in that romance. Rather Yvain is first-cousin to Gawain through their fathers who were brothers. The “Vulgate Merlin” makes out Yvain to be one of Arthur’s nephews, but by another half-sister of Arthur named Brimesent, not by Morgain who is unmarried.
In medieval Welsh tradition, Owein (= Yvain) son of Urien is sometimes the son of Urien by the goddess Modron.
Morgain, in the Charlemagne story of “Huon of Bordeaux” is said to have been the paramour of Julius Caesar and the mother by him of Auberon, a King of Faërie (whence Shakespeare’s Oberon). In the late “Perceforest” romance Morgain is shown to be active in Britain in pre-Roman times.
Some attempt to connect Morgain the Fay to the Irish goddess Morríghan but those more knowledgeable than I about Celtic linguistics mostly dispute a connection between the names. And other than the names, they are not much alike.
Morgain is NOT the mother of Mordred in ANY medieval tale. That appears to be an invention of some 20th century novelists. In medieval romances, Mordred is brother or half-brother to Gawain and their mother is another sister or half-sister of Arthur, never confused with Morgain in romances where both appear. See http://kingarthur.wikia.com/wiki/Mordred . Mordred is NEVER connected to Morgain. Indeed, the only story in which Mordred even meets Morgain is in the “Post-Vulgate Quest of the Holy Grail”, in which he and his four brothers spend the night at her castle. But nothing is made of this in respect to Mordred, save that he and his brothers are all shown the paintings that Lancelot had painted when he was imprisoned by Morgain which tell the story of Lancelot’s love affair with Guenevere.
The name “Morgain” is never mentioned in medieval Welsh Arthurian tales. In the “Story of Gereint son of Erbin'' which retells Chrétien’s “Erec et Enide”, she is replaced by a male physician named Morgan Tud. In a late adaptation of the “Story of Merlin” she is given the very non-Welsh name of Dioneta. See http://www.maryjones.us/ctexts/kaledvwlch.html .
See http://www.archive.org/details/studiesinfairym00patogoog for a somewhat old scholarly book on Morgain the Fay. Like many books on medieval Arthurian tales, it contains lots of theories that are possibly true but also possibly false.