The Nine Sisters first appear as a complete council in the Vita Merlini, a Latin poem written around 1150 by Geoffrey of Monmouth, the Welsh cleric who gave the Arthurian legend its lasting shape. In the poem, the bard Taliesin tells the mad prophet Merlin of an island beyond the western sea called the Insula Pomorum, the Isle of Apples, which men call the Fortunate Isle. It is a place where grain and grapes grow without cultivation, where apple trees spring from the close-clipped grass, and where people live for a century or longer. There, nine sisters rule by a pleasing set of laws over those who come to them from Britain. Each sister has her place in the council, and together they form a government of learning and magic that holds sway over the otherworldly realm.

The sisters are named in Geoffrey's poem, though manuscript traditions vary some of the spellings. The first and eldest is Morgen, more skilled in healing and more beautiful than any of the others. After her come Moronoe, Mazoe, Gliten, Glitonea, Gliton, Tyronoe, and Thiten, who is most celebrated for her cither, a stringed instrument like a lyre. Some manuscripts list variants such as Moroe, Glitorn, Tythonoe, Tythen, and Tithen, but the number remains constant: nine women, nine seats of power, nine voices in the council of Avalon. Geoffrey records that Morgen has taught mathematics to her sisters, so that the whole council is versed in the arts of number and measure as well as the arts of healing and music. They are called nymphae in the Latin, nymphs, and their realm is an otherworld paradise that blends Celtic belief in magical islands with classical ideas of the Fortunate Isles.

The most famous episode in the story of the Nine Sisters is the rescue of King Arthur after the battle of Camlann, the final conflict in which Arthur was mortally wounded by his son Mordred. In the Vita Merlini, Taliesin recounts how he and others carried the wounded king across the western sea in a ship piloted by Barinthus, a helmsman who knew the waters and the stars. Barinthus is the same figure who appears in the Irish Navigatio sancti Brendani, the Voyage of Saint Brendan, as the monk who guided the saint to the Land of Promise. With Barinthus at the tiller, the party reached Avalon, and the sisters received them with fitting honour. Their chief healer, Morgen, placed Arthur on a golden bed in her chamber, uncovered his wound with her own noble hand, and inspected it for a long time. At length she declared that health could be restored to him if he stayed with them for a long while and submitted to their treatment. The king was entrusted to the care of the Nine Sisters, and the others sailed home rejoicing, leaving Arthur in the hands of the sisterhood.

The Nine Sisters are not an isolated invention. Geoffrey of Monmouth almost certainly drew upon older traditions of magical female groups in Celtic and classical lore. The Gallizenae were nine virgin priestesses who lived on the Ile de Sena off the coast of Brittany, described by the first-century geographer Pomponius Mela as women who could cure the sick, foretell the future, control the weather, and transform themselves into animal shapes. Their powers mirror those of the Nine Sisters so closely that the two groups seem to drink from the same well of ancient belief. In Welsh mythology, the nine maidens of Annwfn appear in the poem Preiddeu Annwfn, the Spoils of Annwfn, where they tend a magic cauldron that does not boil the food of a coward. The number nine recurs throughout Celtic tradition, from the nine witches killed by Cei in the Old Welsh poem Pa Gur yv y Porthaur to the nine sorcerous Hags of Gloucester in the tale of Peredur. The Nine Sisters of Avalon belong to this deep pattern of ninefold female power in British myth.

Centuries of retelling transformed the Nine Sisters into a single figure. In the Vulgate Cycle of the 13th century, and most famously in Thomas Malory's Le Morte d'Arthur of 1485, the collective had become Morgan le Fay, Arthur's half-sister, a vindictive sorceress who schemed against the Round Table. Yet even in Malory, the original role of the sisterhood survived: Morgan was still the one who bore Arthur to Avalon after his death, still the keeper of the boundary between the mortal world and the otherworld. Beneath the later legends lies an older stratum of belief. The Nine Sisters bear the stamp of the nine Muses of Greek mythology and the Norse sea-goddess Ran and her nine daughters. What began as a single passage in a Latin poem became one of the most enduring images in Western mythology: a circle of nine women, waiting on an island of apples for a wounded king who might one day return.