Rengoku Hime

Rengoku Hime (煉獄姫) is a light novel series by Yuu Fujiwara with illustrations by kaya8.

Plot
Another world exists one level below the one we live in, the Rengoku. Rengoku is filled with a poisonous gas called "ren." Although "ren" is harmful to the human body, it brought many new possibilities to mankind.

Artemisia also known as Alto is the first princess of the country Ei. She has an abnormal body which is a doorway connected to the Rengoku. Because of this her body emits "ren" uncontrollably, bringing death to the people around her. As a cursed princess, she is locked up in the castle's dungeon, living her life without daylight.

One day Alto met a young knight named Fog and together they received an order from the royalty. Alto's goal is to become a "Rengokushi," controlling the exceptional power that "ren" brings.

"Come, let us dance in the sorrowful Rengoku!"