Seriously, Dark Creepers are my favorite Underdark race because none of the other ones (with the exception of Mind Flayers, my second favorite) are creepy enough and mysterious enough to provide campaign level secrets.
Looking back over every appearance they've had in the books that I own, they seem to have avoided any sort of fleshing out as far as cultural fluff or even motives besides yoinking stuff at night.
Have I just missed out on all of those details? Do they actually have a rich history in D&D?