Story-related achievement. Choose to start the arc by visiting the allegiance map and selecting Grantebridgescire. Few quests later and it'll be finished, and you'll have the town unlocked where you can pick up the rigosogur collectible and do the flyting battle
Answers:
*** Spoiler - click to reveal ***Flyting - Pick any responses to complete.
If you wern't really paying attention then the traitior is Gallin. You still have to do the small mini-quests for the 3 suspects and then report back to the Jarl with your answer.