I am planning to make a custom campaign called Knights and "Merchants - The Southern Invaders." Although I haven't yet created any maps yet; I do have an idea for it's plot.
Knights and Merchants – The Southern Invaders
Volume 1 – Painful losses
Starting off in a farmers village in the south, you encounter some hostile strangers. You battle them but lose eventually. Forced to retreat to a nearby village, you receive reinforcements to reconquer the lost village.
After a successful reconquering a town in the eastern lands requests your assistance. Having to deal with more strange invaders you get to know that more attacks are happening in several places near the south of the kingdom. Eventually the invaders get settled and start to create a legion to conquer the northern lands.
Losing more and more parts of the kingdom you eventually stand a last halt together with all allies that are left. Harsh fighting takes place and the kingdom barely wins. Yet a lot is destroyed and has to be built again.
Volume 2 – Retake and eliminate
Now the northern stronghold has been repaired it is time to reconquer the kingdom. Your first objectives are to build up an army and reconquering the eastern and southern part of the kingdom. Scouts have reported that there are some chieftains who occupy the east are thinking about leaving the invader’s pact. You make your way to the east. When liberating a local farm; you prison an enemy scout who brings a message.
After eliminating the problem of your possible ally, the chieftain agrees to help you reconquer your kingdom. He also tells you that the governor of the capital city is imprisoned in an enemy camp nearby. An executioner squad is on its way to execute the governor. You rush to stop them and liberate the governor in time. He tells you a secret entrance to the capital and lets some forces still loyal to him help you.
As you enter the city’s dungeons you encounter enemy guardians. You slay them and make your way through only to be ambushed. Being in a very bad tactical position, you come out as the victor from the fight. You capture the capital and liberate its folks.
Now your final objective is to eliminate any remaining forces. It is decided that you should first cut off their escape route by destroying the harbor near the village where it all started. Afterwards you mop up with various allied forces for a joint strike on the invader’s headquarter.
Now the last remaining stronghold has been destroyed you discover something new: the invader came from a nearby kingdom which is still occupied. You gather your goods and troops and set sail.
That's my idea so far. If you see some unlogical aspects or if you're having suggestions, please let me know.