Geoffrey borrows the story of the "child without a father" from the Historia Brittonum (9th century). A young prophet is brought to Vortigern as a sacrifice but saves himself by displaying feats of magic greater than those of the King's sorcerers. Geoffrey changes the name of this youth to Merlin from "Ambrosius," claiming that this was another name for him. In Geoffrey he is birthed by an incubus and has no human sire. Found at his birthplace, the town later called Carmarthen, he is brought before Vortigern before whom he reveals two underground warring dragons symbolizing the Britons and Saxons. The Prophetiae ensue. He is pictured as a youth instead of a boy savant. Geoffrey knew little at the time about the person of whom he wrote besides his name and reputation. He places Merlin a full century earlier than his Welsh model and, in a greater anachronism, has him oversee the construction of Stonehenge. He also casts the spell that allows Uther to impregnate Ygerna with Arthur.
Geoffrey returned to Merlin around 1150 in his poem "Vita Merlini" where, having expanded his knowledge of the original bard, he tells Myrddin's tale and attempts unconvincingly to reconcile the dates with those of the Historia. Prominently featured is Ganeida, sister to Merlin and prophetess in her own right.
The Historia however, remains the model for the romancers even though in it Merlin never has any contact with Arthur despite hints to the contrary. He begins to assume a larger role with Robert de Boron. The necromancer is now said to be Devilspawn born with the goal of opposing Christ by the formation of an evil prophet. His mother's virtue is responsible for thwarting this black purpose and the powers bestowed by Hell are turned to the good. de Boron and his Vulgate followers echo Geoffrey by placing him again in the 5th century and including Merlin's dealings with Vortigern and Uther. His deeds are also increased. He is now responsible for the making of the Round Table for Uther as a replacement for the table upon which Joseph of Arimathea placed the Grail. He lives to see Arthur born and sees to his care with a foster family. Lastly, he creates the test that will ultimately prove Arthur's royal birthright, the Sword in the Stone.
The later romancers go even further. Merlin obtains Excalibur from the Lady of the Lake and assists the King in his early struggles for unification. He lays the foundation for the Grail Quest but is seduced by Nimue (Viviane), who steals his magic and then imprisons him with enchantment. Malory takes this version as his end. However, a late Welsh legend has him make a voluntary retreat to an underground wyr or invisible glass house on Bardsey Island. There he guards the Thirteen Treasures of Britain along with the True Throne of the Realm where Arthur will sit upon his return.
Merlin's role as a whole is that of advisor and scholar. His genius guides the realm. Though in modern times he is usually depicted as old, his youth during the events with Vortigern cancel out the possibility of his being much older than forty by the end of his recorded deeds.
The character Geoffrey created combines two people, the 5th century prophet and the 6th century poet. He possibly included a third, a hero or god associated with Stonehenge. Merlin is a happy accident arising out of Geoffrey's lack of knowledge when he composed the Historia. He may not have been too far from the mark, however. There are indications that "Myrddin" was a generic term applied to those in pre-Christian Wales who had ecstatic visions and Divine guidance. Geoffrey may have been conscious of a plethora of Myrddins and, without knowledge of the term, thought they were one and the same. Hence his unification of them into one person. In the 16th century, Welsh chronicler Elis Gruffudd claims the first Merlin was reborn as the second and in addition as the bard Taliesin, another who gains his identity by inspiration. Regardless, Merlin is a cryptic figure who occupies a transitional state, slightly Christian with heavy ties to the older, Druidic world.