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In Warbreaker, Vasher uses Nightblood, an Awakened sword with enough Breaths infused in it that it has sentience. People who draw the sword end up dead and towards the end of the book when Vasher uses Nightblood to cut into a wall the entire thing just turns to smoke. Very similarly, Roshar's Shardblades also have the ability to "sever peoples souls" as quoted in The Way of Kings. Additionally, Shardblades can cut through rock "like a piece of butter on a hot knife".


Is Nightblood a Shardblade? Is there any connection between Nightblood and the Shardblades? Or do they just happen to have a lot in common?

In essence, yes, but of a different degree and even kind.

• 
  

  Nightblood is a "robot spren," whereas Shardblades are ordinary spren. It's an imitation spren created by Awakening.

  
  
  

  
  

  Boogalyhu34: Can Nightblood be considered a splinter and does it

  
  
  

  function like a spren realmatically, are there distinct differences is
  what I'm asking?

  
  
  

  Brandon Sanderson: Nightblood is kind of his own strange thing. He's
  an attempt to use one magic to replicate something in another. He's
  closest to a spren, but kind of like a...robot spren, for lack of
  better words to use.

  
  

  
  
  

  As we can also see here:

  
  
  

  
  

  Cognizantastic: As I understand it, Nightblood is most comparable to a Shardblade. However, unlike a Shardblade, Nightblood requires
  constant input of Investiture in order to realize his full destructive
  potential. Why is this?

  
  
  

  Brandon Sanderson: Vasher kind of hacked in order to imitate another magic system. Shardblades are organic parts of the world they
  are on, but Nightblood is a bunch of souls stuffed into something.
  Nightblood is like a Frankenstein.

  
  

  
  
  

  It was almost certainly an intentional attempt to emulate Shardblades:

  
  
  

  
  

  BRANDON SANDERSON: There are those involved who knew that Shardblades existed before they
  tried the Nightblood experiment.

  
  

  
  
  

  Speculating based on the previous information: the Spren now on Roshar are Splinters of Honor and Cultivation (and sometimes Odium): self-aware portions of Shardic power. We also know that the Divine Breaths of Returned are Splinters of Endowment, and that they're qualitatively similar to many normal Breaths, since they have similar effects. It's not hard to see, in principle, how many ordinary Breaths could be used to create, in effect, a sort of Splinter.

  
  


• 
  

  Nightblood is a "third-generation" blade. In essence, it's more advanced than Shardblades.

  
  
  

  
  

  Brandon Sanderson: Hard to say. They're all similar, but at the same
  time, very different. And in a way, Nightblood is what you might call
  a "Third Generation" blade.

  
  
  

  uchoo786: Ah gotcha. And in this analogy, Honorblades would be 1st
  gen and Sprenblades would be 2nd gen?

  
  
  

  Brandon Sanderson: Yes.

  
  

  
  
  

  In the same discussion, Brandon also talks about how Honorblades (the first generation) are not as efficient at using Stormlight. So there really is a sort of innovation or improvement going on here.

  
  


• 
  

  Perhaps because of this, Nightblood is much more dangerous. Brandon has said that it is heavily invested, far more than even powerful items such as the Bands of Mourning, and second only to Shards themselves. Certainly it is much more powerful than a Shardblade.

  
  
  

  As further evidence of this, Hoid is unconcerned by Shardblades, but is cautious of Nightblood, fearing that it could actually hurt him. Apparently, any rational individual in the Cosmere, including Shards and their vessels, feels similarly.

  
  


• 
  

  Nightblood may have been produced through an unusual process, in which Endowment may have been personally involved (setting aside the enormous number of Breaths and specialist knowledge needed), which could have been responsible for its unusual nature.

  
  
  

  
  

  OrangeJedi: When Nightblood created, was Endowment involved in any
  way more than normal?

  
  
  

  Brandon Sanderson: Good question, you qualified that the right way! I
  would say yes, but maybe not to the extent you're thinking.

  
  
  

  OrangeJedi: Normal being using Endowment's Investiture to Awaken.
  There's something special.

  
  
  

  Brandon Sanderson: I would say, there is something special.

  
  

  
  



• Nightblood is composed of a very different sort of Investiture than the usual Shardblade, which is made of Honor or Cultivation's essences. Nightblood partakes of Endowment, obviously, because of all the Breaths used, and contains (undifferentiated?) Investiture from the things it's consumed. However, Brandon has also hinted at a Connection with Ruin.

Yes, Nightblood is a shardblade. Brandon Sanderson has said so during QAs, such as during a signing for Words of Radiance in San Francisco https://wob.coppermind.net/events/224-words-of-radiance-san-francisco-signing/#e6892

"Nightblood is indeed a Shardblade, basically the exact same thing. Though he is a Shardblade that is twisted and he is a lot more powerful than your standard."