The Whispering Door

After hearing a rumour at a Whiterun inn, talking to the Jarl reveals he is having problems with his son taking a sudden dark turn in his personality. The son says 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 or the Jarl. After acquiring the key and unlocking the door, the voice is revealed to be that of the ebony blade, which is next to a book warning of its corrupting evil.

The quest is completed at this point, but the ebony blade and tells you it will only become stronger via killing, especially if you kill someone who loves you.