It's been ages since I did the main quest, so I don't know if it's a bug, but...
I can't find the banekin that's supposed to spawn to start this quest (Stonefalls). I thought it was near the skyshard, was it moved? Is it failing to spawn for anyone else?
I've been to the Mage's Guild to see if the Tharn's waiting, but nope...
The quest isn't in my journal either. I've got up to Castle of the Worm in my achievements. My level isn't the issue, I'm CP810+.