It's a catacomb located understand from a tribe who has never been outside of the cave and believes Sun is the God. It is located in the highest height to get closer to God. So it is like the heaven for them.
Since they have never been outside, they always wondered what the world beyond "the heaven". One few of them traveled outside and they called Travelers. They told stories about the infinite water land (the sea) including the sea creatures. So people worshiped sea creatures as their guardians of the hero's cemetery.
Here's the process of making this image. I started with simple pen composition sketches and block in model in modo, and did the layout sketches on top.