Once there, a voice tells you that you are more worthy and orders you to open the door. In order to find who has the keys, you must talk to Nelkir again, and he says only the court wizard Farengar Secret-Fire and Jarl Balgruuf carry the key because of the war.
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.
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 does not match its own description, and has fallen to a lesser form due to its aging without use. In order to restore its former strength, it must taste the "Blood of Deceit". For the blade to do this, you must kill someone who trusts you: a person that you did a quest for, or a follower. You can identify people who trust you based on what they say in your presence. They may say something like "Its a fine day with you around", but it's far easier to simply use a follower. Considering this, you may want to use the Ebony Blade in the quest Boethiah's Calling, The Taste of Death, or Lights Out!
The easiest way to do it? Kill a person who trusts you, revive him/her. Do this 10 times and it will be fully