Well I felt in the mood so here it comes, analysis of legend of Piast and Popiel
In XII century
Gallus Anonymous (who was probably Venetian monk Monachus Littorensis) wrote
Chronicle of Poland for
Bolesław III Wrymouth (I wonder if he got the Proud trait). Chronicle was clearly tailored for the customer, as it doesn't mention some things or tend to use allegories in favor of Piast dynasty, but let's get to it!
Gallus writes that once upon a time in Gniezno, it's lord Popiel was throwing a great lavish feast to celebrate his two sons
First haircut (Postrzyżyny). Two wanderers came from far away lands and asked for Popiels hospitality. As he was greedy he gave them none and banned them from his settlement (not very Slavic of you Popiel nuh-uh!). As it happend in the suburbia of Gniezno a not so wealthy commoner Piast was also having not so lavish feast (a fancy dinner, really) for his son's first haircut. Piast was not wealthy but he was kind so he was to happy to have new guests, although the food was scarce. As it happened every time Piast's wife poured wine (it was rather mead, I guess), jug was never empty and no matter how much food she put on table it was never lacking. For the two wanderers were not ordinary folk! They thanked Piast kindly and said that his line would be great rulers one day. Much later, Piast's son Siemowit would throw off Popiel from Gniezno and become a widely approved ruler. Popiel hid in his tower on the lake and was eaten by mice that dwelled it's basements.
Now, what is fantasy, what is half-truth and what is allegory. First of all the story could not happen in Gniezno. Gniezno and other great "baronies" of Piasts: Poznań and Ostrów Lednicki (remember tribe Lendians?) have been built in X century, while the story takes place somewhere around new starting date in Old Gods.
What Gallus is trying to tell us is that Piasts came to be by throwing off Popiel dynasty either as usurpators either as common rebellion or as one tribe conquering other.
Wanderers can actually be christian missionaries. Imagine two guys knocking on Popiels door and asking if he has the time to talk about Lord and savior Jesus Christ and he just shuts the door
No, what I really mean Popiel dyansty or tribe rejected christianity and so it was doomed. While Piast happily accepts them and so his line is to flourish in the new Christian Europe.
Mice are the most fantasy thing in the story and cover what Piasts were ashamed of. Maybe they just killed whole Popiel family or maybe they used foreign mercenearies like viking hirds to conquer his lands. Anyway it's not something to be proud of and does not have a palce in great chronicle of great dyansty.
So where did Popiel rule? There is a theory that he was ruler of White croatian Vistulians in Kraków. Around that time
Svatopulk I of Moravia did in fact conquer land near Kraków, as Vistulan leader rejected christianity. Would that be the turn of events that let Piasts conquer one and for all white croatians south of Greater Poland? Mieszko I did name one of his sons Svatopluk. Or maybe it is just about how Polanie became the hegemony in their federation (Ledianie, Goplanie?)
One way or the other Piasts should be in beginning bookmark of Old Gods, but Gniezno and Poznań should not. I wonder if Silesia and Lesser Poland will be south slavic culture. Even today Polish highlanders from tatry have their quite unique folklore that resembles one from Balkans.