A nice start.
I would suggest the octogonal room to be the entrance (from upstair) deserving a guard room that dispatch to two main part of this underground evil temple On the right, a incantation room where fidels stand in circle to incant ; down this room you can have a kind of clerical stuff room where to place equipment for the mass; north to the circular room, you have the Main Cleric office, a small square room where he/she hides books and secret and the bigger rectangular room at north is a secret place where lies the great altar for demonic mass where only him and a few are able to enter and imprecate...
The left part is a kind of chruch where the north place is the underground cathedral with a hidden treasure room and the "triangle" room is a place where to pray when mass are over. The main corridor from the guard room to the church is for prayer while the small corridor is for officers to enter the cathedral from the transept.
I don't know what lies in the darker place of this underground evil temple, i.e. the strange room with no entrance (magic portal) ?