HypCo says:
Doing a quick read through of the history of caer tan, are the agents of the Thurlahvu strictly dragonborn? If they are, them throwing a party wouldn't work, at least in my mind everyone in the town would be suspicious and no one would go. My thoughts are whomever (the church idea works) heard, sent assassins disguised as merchants or whatever. Taking over the course of a week or so getting antiquated with the town while learning who was the cause of the problem. So they didn't get there and were like 'hey party time boys' could of been a leaving party like 'hey your town was cool. here's a peace out party as thanks'. If any of that make sense. Am I correct that the '__.' on the map are deserts?
Okay, did a total revamp, no where near Eglar. Moved over to Acfeld, where Ambuchar resides, in between Jaha Desert and Porthy Woods. Ambuchar caught words and sent some people, maybe those trying to earn favor with the dragons?
It's true that much of the public presence of the Thurlahvu are dragonborn, as it's hard to ignore chromatic-scaled plate-clad warrior-paladins marching in tandem or guarding shrines. However, there are other elements to each organization that work the shadows, and what easier way to spy on the populace than to pass unnoticed among them?
In other words, no, agents of the Thurlahvu don't have to strictly be dragonborn.
And yes, the line mark within the hex represents desert or wasteland. Acfeld itself is situated more in a marshy swampland, however, and much of the eastern region is covered in ashfall from the active Orm volcano in the center of Caer Tan. It's still okay to originate from Acfeld; being the known court of an ancient black dragon, the governor would show a keen interest in making sure the population stays in check.