During the cemetery level, find a key hidden on a corner of the dumpster. Use this key to enter the chapel and then look directly left and interact with a pipe exposed from the wall. Follow this passage down, then down an elevator and keep going until you reach a room with a fallen beam. Go up the beam and interact with the cross on the left hand side of the wall. This will open a tomb containing the Ancient Sword and your achievement when you pick it up.