Wait, my memory is returning. You go up a long spiral staircase to get to the library; the circle is obvious. As I recall, there is a large cathedral type room where you have a fight with many monsters. Through a door off that room is a room where you meet an important npc. In that room a large spiral staircase.
If that's not it, it's not particularly hidden. I came across it in routine exploring before I knew what it was for. You need to complete that ceremony before you face the demon. Sorry I can't be more specific.
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