"Grand Cathedral" is the location in which you will be fighting the boss "Vicar Amelia". once she is defeated, a lamp will appear.
How to get
Once you have opened the large gate with a special emblem, turn left where an enemy with a "Flamesprayer" is roaming and keep going down, reach the stairs and keep going down still, turn right and go down the alleyway. get rid of the enemies, you will manage to find a ladder, climb it. from there, find your way to drop on a building roof, jump down. (better open the 2 gates which are locked, you will find them easily.) then, go up ALL the stairs until you reach a big door. opening the door will lead you into the room of the boos fight.
Uses
After killing "Vicar Amelia", a skull in front of the lamp will shine as a loot, activate it and then claim the reward.