The description unfortunately fits the entirety of the snowy mountain range that runs along the western edge of Thorne. None would have the unusual marble hands, however he can consider trying his luck and going there. Thorne would allow him to go with two guards since the area is quite dangerous, but they'll request that he return before dark.

Exploring the mountains, there will be little evidence of any vanishings or unusual activity. They might run into any number of dangerous mountain wildlife if you like. While the amount of evidence is lacking, this may imply that the location Sylvain witnessed doesn't exist in an area that characters can yet access. It also suggests the Acolytes didn't go deep into the northern mountains, possibly due to the dangerous conditions or because it isn't terrain that they're familiar with.