Question for the hivemind: How well known is the city of Promise? Everything I read about Mengkare says that he's been working on the city for almost 200 years now. During that time, he sent out the Scarlet Triad (when they still were doing their original purpose) to recruit people to his utopia of concerningly specific perfection.
I'm getting ready to run Book 4 of the AP, and one of the PCs has the background Dragon Scholar from the players guide. Even moreso, the character is a completely obsessed "dragon nerd" who has gone down the path of Dragon Disciple. Would a character like this, with a Master proficiency in Arcana, as well as Expert in Society, know about Promise, given that it is essentially a small country run by an Ancient Gold Dragon? It seems like it is a unique situation, in that nowhere else is there any sort of draconic leadership so openly displayed on Golarion, and a dragon nerd would probably be extremely fascinated by its existence (even hope to go there someday given that they only take "the best").
And doubling down on it, would they know that Mengkare was responsible for The Rending of Droskar's Crag? Or is this second piece of information a more well kept secret (seeing as its entirely possible that very few people, Mengkare included, survived the explosion and consecutive eruption)?
I want to start sprinkling hints about Promise, and Mengkare, NOW in Book 4, so that by the time we get to Book 6, he's not some random character that has never once been mentioned ever, and is suddenly supposed to be the most important thing to the story (which is how the AP is canonically written). I'm thinking maybe the PC's find a flyer when they return to Breachill from Kintargo, advertising a "better tommorow". It doesn't lead to anything currently, but I want to know if I should have them make knowledge rolls, or if Promise is just something everybody has heard of... "you know, the big fancy city run by the Dragon?"
I feel like most well-educated people, would know of the existence of Hermea, but only people with specific knowledge of it or of dragons would know about Mengkare. So yoru dragon disciple player would probably know about his existence, and maybe a bit about his involvement in the dragon plague, but probably not anything about his plans or current alignment other than what can be gathered from the triad
The nation of Hermea is decently well-known, at least among some people, but details about it are generally pretty vague around the Inner Sea I believe. It's semi-common knowledge that the gold dragon Mengkare is running his own island utopia, though what exactly that means is left to rumors. Some are probably curious and excited about what that might be bringing about, a few concerned and suspicious, and some likely just shurg and wave it off as 'another silly dragon cult playing about'.
What would be more easily known - or at least able to be researched with somewhat minimal difficulty once they're asking the questions - is that Mengkare has set up his own nation and city to try and engineer his own perfect community. Only specific recruits are actually allowed to join, and I'd assume it gives off a very 'elitist' vibe to those aware of it.
As for The Rending, I think virtually no one is aware of that fact. Veshumirix is only aware of his involvement by the pure happenstance he was close enough to see Mengkare about in the immediate aftermath, but far enough away that he didn't get killed in the eruption itself.
(Interestingly, while I'm all about foreshadowing in the AP, and you're at the point where the PCs can definitely start trying to piece things together (mine started getting extra curious when they finally found the gold scale near the ring while building around the keep in the downtime between books 3 & 4), recruitment flyers would not likely be the method to do it, unless those fliers are quite old. The fact that Mengkare has pretty much ceased all new recruitment over the last several decades as he approaches his project's completion is the main driving cause of much of the adventure path - it's that lack of need for new blood that cut off the Scarlet Triad's initial revenue stream and started pushing them towards taking their more drastic measures