I have a theory that specifically Geoffrey of Monmouth’s presentation of the Mab Darogan Prophecy in his History of the Kings of Britain was designed so one could argue it was fulfilled by William The Conqueror.
Cadwaladr ap Cadwallon who Geoffrey in the final chapters of Book 12 presents as the last King of the Britons and from whom this prophesied Savior wild descend is nominally a real 7th Century historical figure but his biography is almost completely changed. Others have focused on how the pilgrimage to Rome is actually based on the contemporary King of Wessex.
But what interests me is how Geoffrey ends Cadwaladr’s story with him not in Wales or even Britain. The fact that the medieval Kings of Gwynedd and Princes of Wales descended from the real Cadwaladr not only isn’t mentioned but seems to be something Geoffrey doesn’t want his readers to know. Rather he ends Cadwaladr’s story in Brittany closely linked with a King of Brittany named Alan. It’s likely the medieval audience would have presumed dynastic intermarriages would occur between their lines.
Which is interesting given how Alan’s line was already Prophetically significant. He is in this narrative the direct Heir of Hoel of Brittany son of Budic II of Brittany by Anna King Arthur’s Sister. Back in Book 8 Chapter XV the Prophecy Merlin gives about Arthur also says it would be Uther's daughter from whom the long term line of succession will descend. Now Arthur having sisters seems to be precedented before/independent of Geoffrey, but Geoffrey alone has any sister much less his full sister by Uther marry a ruler of Brittany. The actual Breton traditions about Budic and Hoel do not connect them to Arthur at all. Budic II does take refuge in Britain after losing his Kingdom to the Franks but it was actually in the court of Dyfed. Meanwhile Arthur’s full sister is usually married to Loth by whom she is the mother of Gawain and Agravain and Medraut.
So Geoffrey has two different Prophecies tying the legacy of Sub-Roman Britain to Brittany rather than Wales or Cornwall or Cumbria.
William The Conqueror’s Paternal Grandmother was Judith of Brittany a descendent of Breton rulers including at least one named Alan. And the Dukes of Normandy may have had dynastic ties to Brittany even before her via Sprotta. And William also had Breton allies during his conquest of England.
One of Geoffrey’s patrons was Robert of Gloucester, a Bastard son of Henry I and a supporter of Matilda during the Anarchy. So legitimizing the Norman-Plantagenet Dynasty’s rule over the Island was arguably in Geoffrey’s interests.
Geoffrey is also seemingly the oldest source for a lot of the basis for identifying Arthur geographically with the area of Wessex (I have come to believe based on the older sources that Arthur was mainly in The North). This fits in with the theory that a lot of Arthurian motifs originating during the Norman period were partly about attributing to Arthur accomplishments of Alfred The Great.
Going back to his sponsor’s support of Matilda, I wonder if this is part of the origin of the Cornelia daughter of King Leir legend I discussed in a prior post? Cornelia and Matilda were both married to Kings on the Continent before ruling Britain/England.