This meant my first castle actually belonged to the Kingdom of Rhodoks and they declared war against me instead of the Sarranid Sultanate to get it back. This happened because I captured territory that the faction I was rebelling against (the Sarranid Sultanate) had stolen from the Kingdom of Rhodoks. I also had trouble with a second faction that declared war on my rebellion. None of the other faction's lords would join my rebellion until I had captured two towns and several castles and it was very difficult holding onto all of the territory by myself. I had a lot of trouble in the beginning of the rebellion since it was the first thing I did after winning renown from tournaments.

If this happens, the faction will never disband and you won't be able to get the achievement with this particular claimant. Thanks to Sky Maniac's Empress achievement guide - You need to capture a faction's ruler before capturing all of its territory to avoid a bug where the ruler disappears from the map. At some point later, you will likely be made the marshal for the faction too which would earn you the Autonomous Collective achievement. Your rebellion faction (red color) will disappear and you will become a vassal of the faction you've just conquered. Once they finally disband, the claimant will leave your party and become the new ruler of the newly reformed faction. Capturing them after battles will speed this part up though setting them free will increase your relation with them and hopefully make them more likely to join you. This can take a while as lords still loyal to the faction will continue to roam around and siege your castles for some time, even after the faction has no territory. Capture all of the other faction's territory and their ruler to force the faction to disband. From there you will need to slowly capture the other faction's castles and towns and win over lords to fight for your cause. Once you take up their cause, they will join your party as a companion and you will be the leader of the rebellion against their desired faction.

Travelers in the tavern can give you the location of these claimants and once you have enough renown, you can vow to help them reclaim the throne. For each faction there is a claimant who says that the throne rightfully belongs to them.

Mount and blade warband pretenders