The Great River is a long and narrow inland sea to the south of the Northern Kingdom. It starts in the east where it's fed from the rivers of the eastern mountains and ends at the western ocean some 1000 km to the west. It's narrowness means it's not tidal but there is a slow westerly current to the ocean. The climate is temperate and wet at the eastern end, gradually becoming warmer and drier to the more mediterranean western end.
It's a sparsely populated region of widely spaced, individual city states and townships on the coasts and islands with wild and untamed wilderness in between. There are few good, safe roads and almost everything travels on the Great River itself.
Along the coastline and on the islands in the uninhabited regions are the ruins of long abandoned fortifications and towns, many of which are now home to pirates and goblinoid and orcish bands.