This is a lace sheath by Banana Republic, modeled so stunningly by ExtraPetite.
They're similar enough that if, say, a woman wore the BR and another wore the NL to the same holiday party, they'd get double takes. But the BR one is less than half the price.
I still prefer the NL one, because I love the midnight blue, the trimmer skirt, and the waist-cinching belt (though that, at least, is an easy thing to add). And the NL's lace just looks prettier, a grade or two finer, at least to my untrained eye...but it's hard to argue with "half the price". Especially given the 30% promos that BR rolls around on a not-too-infrequent basis.