In the old times Japan thought one country was full of nothing but darkness. That country was Mongolia. It was mostly because when the Mongolians invaded it was always night time. So everyone just started thinking they were used to the darkness because they lived in it all the time back home.
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
This picture depicts the state at the time of the Mongolian invasion.