The exact origins of the Merovingian dynasty as well as their genealogy.
We know the Franks were a confederacy of germanic tribes that formed around the Third Century. We know the names of some of their early Kings as well as the fact the Salian Franks romanized and served as
feodorati in the Roman Empire. But we have no idea how these various leaders are related and only have hypothesis... We're also not sure which of the characters mentionned in Frankish history are completely historical. And thus we don't know really how the Merovingians came to be.
The first historical ruler we have proof of is Childeric I, father of Clovis I, because we found his tomb and have records about him. But why did he become King? Who were his parents? How is he related to other historical figures? That we don't know for sure.
Supposedly, Childeric's father is Merovee, the King who gave his name to the dynasty and who supposedly fought alongside the Romans against Attila in the battle of the Catalaunian Fields. Merovee is a name that regularly shows up later in the genealogy of the Merovingians so an ancestor of them bearing that name is likely. And we know there were Franks that fought alongside the Romans in the Catalaunian Fields, so there is a historical likelyhood for Merovee's existence to be true. Him being Childeric's father also seems likely given that Childeric died in 481, 30 years after the battle of the Catalaunian Fields: so that leaves room for his father to have been around and fought in the battle. But even with all of that it's pure conjecture: there are no concrete proof to all of this.
And even with that, that doesn't answer where Merovee comes from and why he fought alongside the Romans. Various historians have formed hypothesis that hold some merit because they could explain the level of romanization of the Salian Franks. But it's mostly guesswork trying to reconnect the dots we have. The leading theories is that the Merovingians are descendants of Richomer, a frankish leader serving as a roman officer in the late Fourth Century. Richomer is also the uncle of Arbogast, who himself might be the brother to Empress Eudoxia, wife of Arcadius. This would offer a pretty interesting reason as to why the Salian Franks were so heavily romanized and a reason for them to have had such importance in Northern Gaul given their lineage. But it's highly hypothethical and relies on shaky sources.
To make matters worse, we also have to contend with Legendary or Semi-Legendary figures that were added to the genealogy by zealous writers tying to put the Franks and their Kings on a pedestal. This is how we get the figure of Pharamond, the supposedly very first King of the Franks, who is acknowledge to have been a creation of chroniclers. But to which level is he purely fictionnal? Pharamond is a name we have record of and the way it's form is close to others names you'll find in the dynasty later on... So there might be a historical basis, a real Pharamond that lived and breathed, but either whose name was retaken to create the fictionnal King Pharamond or whose story was overblown to make him the glorious King Pharamond.
But even when you get to Childeric and Clovis, there are mysteries about the genealogy of the Franks. Clovis for example became the sole King of the Franks by eliminating all his rivals, many of which are acknowledged/suspected to be his relatives. But how exactly are they related? There lies a mystery. Hypothesis have been made but like everything I've just talked about, they are only hypothesis.
And finally, you also have to take into account the eventual fate of the Merovingian dynasty and the later period. By the late 7th Century, the dynasty had lost its power and was no longer really ruling the Franks since the true power lied with the various Mayor of the Palaces. Said mayors having a tendency to get a Merovingian out of a monastery to crown him King... This makes following the lineage of the various late Merovingian Kings a bit of a challenge. No one knows for sure for example where the last Merovingian King, Childeric III, comes from: was he a son of Thierry IV? Of Dagobert III? Of Chilpéric II? Of Clotaire IV? All we know that he was taken out of a Monastery by Pepin the Short to placate the Frankish aristocracy before Pepin found a way to make himself King of the Franks... and sent Childeric and his son Thierry back into a monastery as a result.
Pure Folklore, seems was Wishful thinking of a British Julius or Augustus on a fashion, as Britain would have It Rough in the post roman era.
To my understanding, King Arthur is admitted by historians to be more of a composite character of various things brought together. It's likely you had several briton leaders that fought in the war against the Saxons for example and we have definite proof their conquest of England was stopped for a while. This got translated later into the idea that there was a King that united the Britons against the Saxons and defeated them repeatidly, but most likely it's more that various briton chiefs were fused together into the figure of Arthur. The name itself might come from one of those leaders, which is why you have hypothesis about who the "historical" Arthur is.
So it's not really pure folklore: parts of the Arthurian legends have a historical basis. It's just that we have too little sources and archeological traces to know the exact historical truth and determine exactly where the figure of Arthur comes from.
Later additions to the Matter of Britain of course don't help because Arthur got turned into this great heroïc figure and the ideal King rulers should be modelled after, whose knights went on many adventures and fought off monsters. But that also happened to Charlemagne with the Matter of France, and we know Charlemagne was a historical character. So it's possible Arthur got a similar treatment to Charlemagne... We just don't have as many sources on the former than we have on the later to know fact from fiction.
More like, did the conversation really happen or was it fabricated hagiography?
Don't we have records that show Attila did meet the Pope? And that he did turn away after that?