It says they are only encountered in Gehenna, but I found one on the bottom level of the Gnomish Mines (catacombs), it was invisible and it cursed several of my items.

However I suppose it could have been generated by the polymorph trap I found on that level.—The preceding unsigned comment was added by 24.18.219.255 (talk • contribs) .

You just answered your own question. Not only that, someone came up with this on Talk:Vrock. If we had a FAQ, surely this would merit an entry: a tough monster in a not-tough level usually indicates a poly trap around somewhere. I like to spring any poly traps I find as soon as it is safe, for just this reason.--Ray Chason 18:34, 29 December 2007 (UTC)