What Is The Devourer?

When dropping Nell Bund off at Lowtown, there were groups of people cloaked in pure black robes moving through the crowd. Midna suggested finding some black robes and trying to infiltrate the group. When they finally meet, the group is even sketchier in person. They claim to be followers of Liguaquide, that they are helping the town, and blessing the water. When the group attempts to heal the rown, they flee, but not before issue=ing a strange warning: The Devourer will have us all.