Nightblood leek

[Wits instant noodles]

Nightblood is seen to leak investiture in the form of black smoke. does this smoke have any affects on anything? What if Nightblood were to be destroyed would the investiture slowly leake out or would it form a kind of blast of stored investiture?

[Quantus]

It's leaking Investiture, and one WOB implies that it might be connected (or Connected) to Ruin's Investiture, so somethign akin to the Black Mist. 

So far it hasnt been shown to do anything directly, though nobody has tried anything exotic like fueling a Surge with it.  

However, once the smoke is leaking Nightblood enters an Unleashed State, where his personality changes to a far more destructive and shouty version, and where even the Aluminum Sheath becomes part of the Weapon.

 

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Walin

Does Nightblood contain any of Ruin's Investiture? Like, not atium, but...

Brandon Sanderson

Yes, technically; and I'm not wiggling around that, because technically, location in the Cosmere and who belongs to what gets really weird, right? Because Ruin's Investiture is everywhere--but I'm not talking that way. I'm talking the way you actually mean it. 

Legion Release Party (Sept. 19, 2018)

 

 

 

 

Quote

 

Brandon Sanderson

Nightblood

Nightblood's name, by the way, is supposed to sound kind of like the names of the Returned. I played with various different ways for his powers to manifest. I liked the idea of him driving those who hold him to kill anyone nearby. It seemed to work with the concepts that have come before—a kind of unholy, sentient mix of Stormbringer and the One Ring.

The strangest thing about him is the idea that his form isn't that important. The sheath is like a binding for him, keeping his power contained. So drawing him out isn't like drawing a regular weapon, but rather an unleashing of a creature who has been kept chained.

Once that creature is unleashed, he becomes a weapon—even if he's unleashed only a little bit. The sheath itself turns into a weapon, twisting those around it. You don't need to stab someone with Nightblood to kill them; smashing them on the back with the sheath works just as well. It will crunch bones, but beyond that, merely touching them with the sheath when the smoke is leaking can be deadly.

Warbreaker Annotations (Feb. 7, 2011)

 

 

 

[Kandrafish]

Based on the WoBs above there is a definite correlation that smoke means that the blade is now deadly, whether this is cause and effect or effect and cause is uncertain. 

However, also based on the quotes above, the smoke seems very similar to someone reaching a "mist point" level of power and is likely a result of the numerous effects that Nightblood has. All of the effects must be investiture costly. However it could be the smoke that gives the power, it seems unlikely based on other magic systems, though. It is most likely like the mist from Mistborn or the frost from Stormlight.