Oh, ah... I guess you wouldn't know what the sword is. It's one of the church's relics, and it was originally in the Holy Tomb below Garreg Mach, where Sothis was laid to rest. Really, you could say the entire monastery is just one big facade to guard the tomb.

[How should he put this...] The sword is missing a Crest Stone, so it was useless to most... except you. Knowing what you know, I'm sure you can guess why that is.