Slaeter's Sea is at one end of the River Vid, accessed via blue portals found in the Dungeon and sometimes in the forest. On your first arrival, you will always find the artifact Skidblanir next to the portal. The sea has a number of islands, and a narrow coastline. Skidblanir will help you navigate the screen, but the monsters can be a problem:
There is an artifact in one of the water squares. A potion of depredation will help you find it. You can retrieve it if you have the skill of swimming, or you can terraform that water square into dry land. To the left exit of the screen is the River Vid, to the north is River Gioll. Exiting the other ways will take you to the World Sea.