Japanese Empress
noun, の adjective
The emperor empress is the emperor's wife. That's the Japanese Empress, specifically, since 皇 usually refers to the Japanese emperor.
こうごう
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own. Be careful of the rendaku.
彼は誤って皇后であった母親を殺害してしまいました。
He accidentally killed his mother, the Japanese Empress.