It is simple if you already have the spear go to the front of the Cathedral where the blue
door is and stab the WHITE lions statues the chest will appear then.
PS
After purifying the inmortal grab the coffin where the three wight switches are put the
coffin in one, the wooden crate in the other, and stand in the last one the middle lattice
door will open, now you can grab the blue key to open the blue door mentioned before so
you can get out faster and you'll find three weapns there, too.