Noun and の adjective
こうごう
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.
The emperor empress is the emperor's wife. That's the Japanese Empress, specifically, since 皇 usually refers to the Japanese emperor.
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.