hmmmm....  (thinking while typing, could be rambling. ;) )  something to do with the Forbidding?  Or something else entirely?  Maybe one of the other sets of elfstones is thought to be in troll country?

Though I also think runes and rocks when I think of trolls.  Sometimes when I'm stuck for ideas, I like to browse deviantart.  This is kinda cool:  https://www.deviantart.com/fibacz/art/Rune-Circle-76564900

Though I'm not sure exactly what it would do.