After defeating the Claw of the Shadow and the Blade of the Shadow that appear at the final area of the Lair of Shadows map (last area of the Bottomless Pit map), you should find a statue that is similar to singing statues. There, you should obtain the shadow stone.
Once you get the shadow stone, leave the area and go back to the Ship Graveyard map. From the Forest of Ships area, use either of the exits to go to the Sea of Spires map. From the Sea of Spires, find your way to the top and exit.
You can use the screenshot of the map below for guidance :
Once you’ve entered the area, you should be in an unchartable area. Keep moving forward and you should unlock the Spiritfall map, followed by the Summit of the Gods map.