The Whispering Door

Background
"Balgruuf the Greater, Jarl of Whiterun, has a troubled child. He's asked me to help the boy."

Walkthrough
After hearing a rumor at the Whiterun inn (The Bannered Mare) talking to the Jarl Balgruuf in Dragonreach (or if you sided with the stormcloaks in the civil war he is in the basement of the Blue Palace) reveals he is having problems with his youngest son Nelkir taking a sudden dark turn in his personality. Nelkir tells the Dragonborn how a Whispering Lady tells him secrets and he directs you to the locked room in the basement.

Once there a voice tells you are more worthy and orders you to steal a key from either the court wizard Farengar Secret-Fire or the Jarl. After acquiring the key and unlocking the door, the voice is revealed to be that of Mephala, who instructs you to take the Ebony Blade, which is next to a book warning of its corrupting evil.

The quest is completed at this point, but Mephala tells you the Ebony Blade will only become stronger via killing, especially if you kill someone who loves you.

You can pickpocket Farengar through the map board in the side room.

Waiting for either Jarl Balgruuf or Farengar Secret-Fire to go to sleep allows you to pick-pocket them without worry as long as you are Hidden.

Bugs

 * If you upgrade the recieved Ebony Blade at a Grindstone, after making it eat a soul, the damage will be lowered and you will be unable to upgrade it again.
 * If you sided with the Stormcloaks in the civil war and the Jarl has relocated to Solitude, his son stays in Whiterun for some reason.