Am I doing something wrong?

I'm currently trying to do the "Balance of Darkness" quest to reach Dark Acolyte rank. But for many runs I make in mau 4, 5 and 6, I can't seem to find that altar. Just found something looking blue and shiny in the center of the place where Lich Corvis spawns, but it does not give me an opting to click on it or anything, so I am a bit lost on this one...

Would you be so kind to show me or describe how to find these altars and how do they look like, plz?

Ty for this awesome guide