Talk to Catelyn in Booty Bay.
The mage that is in possession of the Stone is amplifying its power through some focal point... an altar of some sort. You should be able to disrupt it by overpowering it.
As it happens, I know just the thing: an ancient dagger, another troll relic. There's just one... little problem.
You see, I found the dagger while researching in Stranglethorn, but I gave it to my willful daughter, Catelyn, who ran off to Booty Bay to become a pirate.