There are six pieces of Agrippa's broken orb which must be assembled in order to advance on with the story. Three pieces of orb are found in the Choir, and three pieces are found in the Transept.
Transept Pieces: In each of the torture rooms (Brass Bull, Iron Maiden and the Wheel). The orb pieces are all found on the ground, so scour the room for each one before moving on to the next.
Choir Pieces: Again, all three pieces can be found in the torture rooms (the weight room, the sawing room, and the triangular chair room). The orb pieces are sometimes found on the ground, but may also be in cabinets.
Once all six pieces are collected, Daniel can go to the Chancel to advance with the story. Even with all of the orb pieces in the inventory, this achievement will not pop until you actually combine all the pieces and form the entire orb while gaining entrance to the Inner Sactum.