According to Istania and Arzagos, these two camps have the missing pieces of the Dragon Banner. You will then be notified of two bandit hideouts near the main settlements (seen below).
I suggest you talk to both of them before selecting which side you will be taking on. Arzagos, meanwhile, lives in Mannurath he prefers that the empire be destroyed and that a non-imperial faction should rule in its place. Istiana is in Epicrotea and she wants to continue supporting the Calradic Empire despite its fracturing due to civil wars.
These are the two important characters who’ll tell you about Calradia’s history. You will be tasked to speak with both Istiana and Arzagos. This is activated as soon as you completed the first main quest “Neretzes’ Folly”. Useful Tips for Main Quests Assemble the Dragon Banner This guide like walkthrough will help you on Assemble the Dragon Banner Quest and Istania’s Quest in Mount & Blade II: Bannerlord.
