Cespelatus

Two nations: one a merchant republic with an unparalleled navy, and the other a hardy military empire are symbiotically linked together by the fact that they physically cannot invade each other - the naval nation has too many navigable rivers on its borders for a land invasion to be possible and the land nation has too many troops for anyone to invade them. A group of adventurers looking to make a new start arrive in the merchant empire and begin to hear rumors of a powerful force approaching from the north that may be just strong enough to disrupt the fragile balance of power between the two nations.