Mongolia
proper noun
The region of darkness in days of old referred to Mongolia. At least in China, where this word comes from. Imagine all those ancient Mongol tribes at the dark, unknown edges of China. Everything beyond the borders was shrouded in (figurative) darkness, I guess. No wonder they picked these kanji back then.
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
この絵には、蒙古襲来の時の様子が描かれています。
This picture depicts the state at the time of the Mongolian invasion.