The Whispering Door

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

- The Dragonborn

Walkthrough
Note you must be at least level 20 and have finished Dragon Rising to receive this quest. After hearing a rumor at the Whiterun inn (The Bannered Mare) talk to 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, who has been 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 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. 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.
 * 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.

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 and The Taste of Death.

The easiest way to do it, is to kill a person who trusts you, revive him/her. Do this 10 tmes and it will be fully upgraded.

Bugs

 * If you upgrade the 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 have started the quest "The Fallen " and have asked Jarl Balgruuf the Greater to be prepared you cannot question him until that quest is done.
 * After speaking to the innkeeper at the Bannered Mare and receiving the quest to speak to Jarl Balgruuf the Greater, sometimes unable to get the dialogue option to ask him about his son. No known solution.
 * Either starting or finishing the quest, or picking up the Ebony Blade may cause your game file to become corrupted to the point where any subsequent saves will never re-load (confirmed PS3). Because of this, and because there doesn't seem to be any known solution, it's advisable to make several separate game saves, one before starting the quest, one before opening the door at the end of the quest, and one before picking up the blade itself.
 * Placing Ebony Blade on a weapons rack can cause it to disappear, losing it forever.
 * It's possible to open the door the whispering lady is behind without console commands. Simply get Whiterun Guards after you, and go very close to the door. They may open it, but it is impossible to enter, as the whispering lady (which the model for is simply a box) is floating in mid-air, blocking the way.
 * Sometimes, the door won't open, even if you have the key (it will only say the key is required). Making the quest unfinishable.
 * If you marry/get wedded then you can kill your wife to get full power or if you kill 5 people you will fully charge it.How to get a Wife.
 * The Ebony Blade regenerates your health from every swing you do on your enemy. This weapon is good if you run at an enemy (Purposedly weaker or not to higher than yourself) and you keep hitting them until they are dead. If you have used the Ebony blade correctly then you should have lost Minimal - No health.