Hi tylfly,
Do you know the origin of the live rock? I don't recognize the species, so it's either an uncommon variant of an Indo-Pacific shell, a rare species, or it's from someplace else. It could possibly be a muricid (it resembles a couple of the Cytharomorula/Morula species), but I'm pretty sure it's a buccinid whelk, and probably a Pollia/Cantharus species. The closest one I could find on the net is the top shell on this page:
http://www.gastropods.com/7/Shell_3237.html
George Sanguiolglou probably wasn't sure what species it was, and decided it was closest to Cantharus wagneri, which it isn't, even though it's listed on that species' page.
I found another similar shell figured on Plate 241 in Okutani's "Marine Mollusks in Japan," labelled as Cantharus fumosus. I've seen plenty of C. fumosus from different locations, but I've never seen one as lightly colored as the one on that plate, or as lightly colored as the one in your pics.
If it's not a variant of one of these species, it's probably a different species of Cantharus or Pollia. In any case, it will be a predatory snail, and will eat other snails in your tank.
I'll keep looking and see if I can find a positive ID.
Cheers,
Don