These creatures are foul things that prey on the weakness of loss. Their home society, and indeed, their home, in completely unknown, as all that have been found have been killed to out standards.
What if the sources of precious metal in the realm all failed, so that the only sources for gold and other precious metal were hostile foreign lands? Gold coinage might become increasingly rare, resulting in hoarding. Player characters that appear with masses of treasure might be suspected of being in the pay of foreign powers.