This is a story of humanity that has lived underground for many generations, so long that no one knows what light is.
This underground society is vast and has many cities and cultures, as well as a leviathan.
This story starts off with a boy who is orphaned in a fishing village. This boy seeks revenge against this leviathan that killed his father. He trains and prepares to hunt and kill the leviathan but the village people tell him it's to dangerous and to leave the leviathan only for it only takes a little here and there. They didn't want it to come and kill them all. But the boy doesn't care and one day years later the leviathan comes. The boy runs and challenges it and by sheer luck and determination the boy kills it. The boy is praised by all and his fame spreads. The boy does not wish to stay but to leave the village.
Next the story shifts to a young woman and she is a priestess. She is a political leader and there is a possible war against an empire that enslaves people. She must forge an alliance with a major figure to rally the people's support for the war. She hears of a young man who is admired by the people for killing a leviathan and she is hesitant to form a alliance with him for he is an unknown.
They meet to form a alliance and to finalize the agreement they marry each other.
Now after this my memory is less sharp and there is a point where war happens and she is taken as a slave into the enemy empire and he fights in the war to find her. She is trained by a prominent family and she later escapes back to him. Both of them and many refugees leave upward and finally find a way outside and the book ends with everyone experiencing first the night sky they think is very bright and then the sun rise.
This story I thought was unique but I have forgotten the name and author. It's a long story.