It is a miscellaneous quest received from Adrianne Avenicci in Whiterun. She asks the Dragonborn to deliver a sword that she forged to her father.
Go to Dragonsreach and meet Proventus Avenicci, tell him what his daughter wants, and give him the sword. In return, he offers 20 he offers as a reward.
If Whiterun is captured after siding with the Stormcloaks during the Civil War, then Jarl Balgruuf the Greater and Proventus Avenicci reside in the basement of the Blue Palace in Solitude.
It is possible to steal the sword from Proventus Avenicci after he receives it. However, he will likely have equipped it, thus requiring the Misdirection perk to pickpocket equipped weapons.
- 360 Sometimes, the greatsword remains stuck in the inventory. Completing or starting the Diplomatic Immunity does not remove it from the inventory.