A simple mod satisfying my need to min-max cultures, religions and other stuff in CK3.
A Culture that prospered in the lush lands near rivers and centers of trade. Due to their prosperous core lands, they are able to fuel grand armies of heavy cavlary to dominate anyone in their way.
Being forced to the undesirable Lands of the Marshes, over generations they adapted to their new home of mud and water. Those lands also protect them from their often better equipped enemies, and their Longbows strike far and hard.