The Austrian monarchy was losing its grip in Europe and it seems some royals lost touch with reality as their powers waned and they sought out a make beleive world to justfy their thrones.
Not sure but sounds interesting.
THE HERESY MYTH :"A number of legends associate the Merovingian kings with the royal bloodline of Jesus and Magdalene. One of their myths is that the ancestress of the Merovingians was a mermaid and another says that the mother of Merovee' was impregnated by a sea monster. In each of these myths, the prevailing kernal of truth seems to be that this ancestry is "half man, half fish." Since Christ was known to early Christians as the "ICHTHYS" and Mary Magdalene was identified with the shape known as the "Vessel of the Fish,"
The above blasphemy was copied from a convert to Merovingians belief they are descendents of Jesus and his relationship with Mary Magdalene. One of early Kings helped perpetrate this lie claiming he was the descendent of that relationship. Holy Blood, Holy Grail is book that spreads that lie.