Temenos x Throne Intermission Cathedral

I entered with intermission chapter with light feelings as they were talking about stealing a treasure and you know that the combination between the thief Throne and the priest Temenos is kind of funny.
It ended up being way more plot heavy than I thought. What are they talking about here? Who is this woman sealed in the cathedral by the previous bishop? Why does she think we have a fire in us? Is it related to the overarching story (you know the recurring Galdera thing) or...? I'm quite confused right now as this came out of literally nowhere as a curve ball.
Just how much stuff was the bishop hiding? Before he died he was seemingly involved with so many plots across the different characters...