The Lunar Pool
The Lunar Pool is a small cistern located in the tower atop St. Jenel's in the Old Hammerite Cathedral grounds.[1] It was built by Brother Renault before the Cataclysm in an attempt to create a substitute for Holy Water. Using his optic lenses and instruments, he succeeded in trapping the light of celestial objects within a body of water. Although it did not have the intended properties, it was found to possess the ability to bless holy symbols in the absence of a Hammerite Priest.[2]
Note[]
I could tell you where it is, but that would spoil all the fun...
This page contains spoilers! Read on your own peril.
The Lunar Pool is needed in the TDP/TG mission "Return To The Cathedral", where Garrett must perform the consecration rites for Brother Murus in order to escape the cathedral.[3]