Some of the earliest tales of King Arthur come from the Welsh poets, and Arthur and his men appear in other tales, such as "Culwch and Olwen," which are not specifically Arthurian stories. Thus Arthur has his place in the Celtic mythic structure, and many elements of Celtic mythology accrued to his legend and became inseparable from it. Although over the centuries Arthur became less a Britonic chieftain and more a medieval Christian king, numerous Celtic themes and motifs still remain in later accounts. Le Morte D'Arthur by Sir Thomas Malory is one such example of a work which purports to be wholly Christian in nature, but which keeps, probably unconsciously, a number of Celtic pagan elements. Particularly in the early parts of the work, Malory's retelling of the Arthur legend reads much like a Celtic legend, with Arthur as a typical Celtic hero-king.
Even before his birth, typical Celtic motifs begin to appear which show Arthur to be in the mold of the ancient Celtic heroes. His conception, like that of all heroes, takes place under unusual circumstances, when Uther enters Tintagel Castle to sleep with Igraine. Merlin assists Uther by magically disguising him as Gorlois, Igraine's husband, so that he might enter Tintagel unhindered while Gorlois is in battle (Malory 11). This gives the conception of Arthur the necessary element of the supernatural while also utilizing two common Celtic birth story motifs: the secluded woman and the shapechanging father (Rees and Rees 236). Igraine is confined at Tintagel so as to be inaccessible to her prospective lover, much like Ethne, mother of Lug (Rees and Rees 214). In the story of the conception of Mongan, a king of Ulster, Manannan mac Lir, an Irish deity, sleeps with the wife of Fiachna Finn while in the shape of Fiachna specifically to conceive Mongan. Manannan takes Mongan when he is but three days old and raises him in the Otherworld, much like Merlin takes Arthur to be raised by his choice of foster parents until he was ready to assume the kingship (Rees and Rees 221; Malory 13-14).
The conception of Arthur follows immediately upon the death of Gorlois, and while it would be hard to ascertain an exact mystical cause-effect relationship between the two events, the conjunction of the two occurrences echoes another motif in Celtic birth stories. In many Celtic legends, the father or grandfather attempts to prevent the conception of the child or, if that is not successful, tries to kill the child because of a prophecy that the child will cause his death or the loss of his power, either directly or indirectly. The story of the birth of Lug is one such tale, in which Balor confined his daughter to a high tower, and when through druidic magic Mackinealy managed to reach her and impregnate her, Balor attempted to drown her three children. A similar event took place following the birth of Finn mac Cumhail. It had been prophesied that Cumhail would die in his first battle after his marriage, and that his son would take the land from the current king. After Cumhail's death, the king attempted to have Finn drowned, and when that failed and Finn was hidden, ordered all male infants to be killed, again to no avail (Rees and Rees 214-15).
These stories offer a striking parallel not only to Arthur's own birth and the death of his mother's husband, but also to the birth of his own son, Mordred. Arthur sleeps unknowingly with his own sister, Margawse, and on her begets Mordred. Merlin later reveals to him that Mordred will cause the destruction of Arthur and all the knights of his realm at the same time he reveals the secret of Arthur's own heritage, after which Arthur has all children born on Mayday killed to try and destroy Mordred (Malory 58). Although this is a common mythological motif, Malory Christianizes it by having Merlin declare the anger of God at Arthur's sin of lust and incest (Malory 47). Conception through incest is yet frequent occurrence in the stories of significant Celtic births, such as the births of Daolgas, Conchobar or Lugaid of the Red Stripes. The latter was fathered unknowingly by three of his mother's brothers, and in his adulthood fathered a son on his own mother (Rees and Rees 234). Thus the incestuous conception of Mordred takes on a mythological significance as well as a Christian one, showing the importance of his birth while at the same time bringing down the wrath of the Christian God upon Arthur.
Merlin serves as a prophet for the word of God, but perhaps more interestingly also serves as an Otherworldly guide for Arthur when dealing with the supernatural. Merlin himself is clearly a supernatural creature in his own right, as shown by his magical powers, mysterious knowledge, and his frequent association with supernatural motifs and connections to the Otherworld. For the Celts, the Otherworld was the realm of the supernatural, whence came people and creatures with unusual attributes, particularly women. The Otherworld is separate from this world, but can be reached by means of animal guides or various symbolic boundaries. Boundaries between places, between water and land, between times of day or seasons, between waking and sleeping or between life and death are all indications in Celtic myth that the Otherworld is near to this one (Rees and Rees 92-4). At numerous points in Le Morte D'Arthur several of these Otherwordly motifs come together, often at a significant event, and are likely remnants from earlier Welsh or Irish sources that came down to Malory. Merlin appears in several of these instances, such as the aforementioned revelation of the true parentage of Arthur. In that scene Merlin uses his supernatural powers to disguise himself first as a young boy, then as an old man (Malory 47). He thus embodies the entire birth-to-death fertility cycle, which is another Celtic theme connected with the Otherworld. Merlin also tends to be associated in Malory with boundary lines, as when in the beginning of the book Merlin makes a point of crossing the threshold of the pavilion before speaking of his magical knowledge and abilities, almost as if crossing from the Otherworld itself (Malory 11). Later on, Merlin advises Arthur to send for help to the kings beyond the seas, Ban and Bors, who cross the water at Arthur's summons (Malory 24-5). They arrive at the court on the eve of All Hallowmass, which in the Celtic calendar is the holiday of Samhain, a time when the boundaries between the worlds are especially permeable (Rees and Rees 89-90). Although Malory no doubt saw them as merely being allies from France, their association with Merlin and the other markers of Otherwordly involvement could possibly indicate an original Celtic root story in which Arthur received true Otherwordly assistance. A similar situation occurs in the Welsh story of Pwyll, Prince of Dyfed, when Pwyll is approached by the Otherwordly king Arawn, who asks for Pwyll's assistance against another king who is making war upon him in the Otherworld, similar to the problems of Bors and Ban with King Claudas (Ford 38-9).
Another highly common motif associated with the Otherworld in Celtic myth is women, especially women associated with horses or water. There are many examples of Otherwordly women in the Arthurian tale, most notably the Lady of the Lake, who gives Arthur a sword in exchange for a gift she will name later. She is clearly a supernatural figure, since she appears from the middle of a lake, and what she gives Arthur is more than a sword. He cannot be king without a sword to use in battle, and therefore by giving him the sword she is also giving him sovereignty. Sovereignty figures such as the Lady are a major Celtic motif, as in the story of Niall. Set apart from birth from his step-brothers, Niall encounters an old woman by a well, who asks him and his brothers each for a kiss. His brothers all refuse her, and only Niall complies by kissing and embracing her. She then transforms into a beautiful woman who grants Niall kingship over Ireland (Koch and Carey 194-6). Thus she grants him sovereignty in exchange for symbolic fertility just as the Lady of the Lake grants Arthur sovereignty in return for a future service. She later comes to Arthur on horseback to demand her price: the heads of Balin and the damosel with the sword. Instead Balin beheads her and declares her to be a sorceress, bringing shame onto both himself and Arthur and breaking the terms of the sovereignty agreement (Malory 64-5). Mythically this could be seen as another reason for the eventual downfall of Arthur and his court.
No one can know for certain how much Sir Thomas Malory knew of Welsh and Irish legend, nor which elements of his work came by route of which sources. Yet it is apparent that many elements of Celtic myth remain in this version of the Arthurian saga, and continue to influence interpretation of it even today. Understanding the specific motifs and themes of Celtic stories help us achieve a greater knowledge of the mythic structure of the Arthurian legend as a whole and how it has developed through the centuries.