Skyrim:Ebony Blade

The Ebony Blade is a Daedric artifact found during the quest The Whispering Door. It is an artifact of the Daedric Prince Mephala. Essentially, it is a two-handed katana (with the speed, length, and reach of a standard, one-handed sword) with designs similar to normal ebony weapons.

Upon obtaining the artifact, Mephala bids you to unlock the blade's true potential by murdering your close friends (typically, those you have done favors for). For every two such NPCs that are killed with the blade, Mephala will speak to you, and the blade's Absorb Health effect will increase by 4 points. After a total of ten people are betrayed, the absorb health effect will max out at 30 points per hit, and won't become any stronger.

  Suggestions for charging the Ebony Blade can be found here.
 
Related Quests
 

Powering the Blade
Mephala will speak to you after the blade has been powered up to the next level using the blood of deceit, to congratulate you on your progress. The line of dialogue that she uses is random.
"Excellent work, child."
"I can feel their heartbreak swelling in my blade."
"Their tears shine the ebony to a sharpest gleam."
"Ah, the blood of deceit is a nourishing flow."
When you have claimed your tenth and final victim, Mephala will be overjoyed and speak to you one last time: "At last, my blade is returned to its full glory. Now go forth, child. Continue your tiny subversions against the orders of trust and intimacy."

Notes
Although any NPC with a relationship value of 1, 2, or 4 will power up the blade when slain with it, some are more suitable than others. To minimize the repercussions of enhancing the blade, it is often beneficial to choose targets with replaceable functions, such as the owners of mills and mines, or those who must be slain during particular quests.
A good way to determine whether or not an NPC is considered a "friend" is to listen to how they greet you. People that say things like ''"You've been a good friend to me" or "It's a fine day with you around"'' are viable targets for powering up the Ebony Blade.
 An NPC's relationship value can be determined by the use of the console command  player.getrelationshiprank   , or alternatively click on the NPC so that their RefID is showing and enter  .
 You can check your current progress towards enhancing the blade by entering the console command   and looking at the "FriendsKilled_var" line.
The boosted absorb health effect functions based off a perk. As normally only the player has the perk, any followers given the Ebony Blade will only be able to use its base stats.
A second Ebony Blade (item ID  , DA08RealEbonyBlade) is included in Skyrim, though it cannot be obtained without the use of the console. This version of the Blade allows you to attack NPCs without having them become hostile. This effect matches the description of how Mephala says the Blade should function once it is sufficiently powerful. Murders using this blade will still give you bounty if you are witnessed.
Though every other Daedric artifact weapon is able to be improved through smithing, the Ebony Blade is missing the required data to be tempered at a grindstone. It is also missing a classification for weapon material, preventing it from benefiting from Smithing perks. This is perhaps explained by Admonition Against Ebony which states that " ...Not even the hottest fires of the Skyforge could melt it ..." indicating that the Ebony Blade is immune to the effects of heat from any source. The sword has infinite magic uses and does not require recharging, which is some compensation for the lack of physical damage increases.
Despite Farengar being allegedly aware of the Blade, you can actually sell it to him.
You don't need to power up the Ebony Blade in order to gain the Oblivion Walker achievement.

Bugs
 
 The Unofficial Skyrim Patch changes the weapon to behave like an Ebony Greatsword in all respects, except for its weight, which is changed to 12. 
 
 
This will always happen during Diplomatic Immunity.