Why did Moash have to kill Jezrien?

[Seloun]

So, doing a re-reading of Oathbringer, I noticed the following passage:

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Shallan knew Re-Shephir at the same time as the thing came to know her. The spren tugged and prodded at Shallan’s bond with Pattern, seeking to rip it free and insert herself instead. Pattern clung to Shallan, and she to him, holding on for dear life.

...

The creature trembled, and Shallan finally saw the reason for its fear. It had been trapped. The event had happened recently in the spren’s reckoning, though Shallan had the impression that in fact centuries upon centuries had passed.

Re-Shephir was terrified of it happening again. The imprisonment had been unexpected, presumed impossible. And it had been done by a Lightweaver like Shallan, who had understood this creature.

Sanderson, Brandon. Oathbringer: Book Three of the Stormlight Archive (p. 310). Tom Doherty Associates. Kindle Edition. 

There are two important things to note here: first, Re-Shephir attempts to bond with Shallan. Second, it was a Lightweaver that trapped Re-Shephir.

And we know that Melishi trapped Ba-Ado-Mishram. The general assumption, based on the WoR epigraph, was that this was something Bondsmiths could do:

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So Melishi retired to his tent, and resolved to destroy the Voidbringers upon the next day, but that night did present a different stratagem, related to the unique abilities of the Bondsmiths; and being hurried, he could make no specific account of his process; it was related to the very nature of the Heralds and their divine duties, an attribute the Bondsmiths alone could address.

Sanderson, Brandon. Words of Radiance (Stormlight Archive, The) (p. 675). Tom Doherty Associates. Kindle Edition. 

But perhaps it was not that Bondsmiths could trap the Unmade, but instead, it was something specific to Bondsmiths and Ba-Ado-Mishram specifically. We know that during the False Desolation, BAM was exhibiting Bondsmith-like Connection abilities. And we know how modern fabrial techniques trap spren:

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First, you must get a spren to approach.

The type of gemstone is relevant; some spren are naturally more intrigued by certain gemstones. In addition, it is essential to calm the spren with something it knows and loves. A good fire for a flamespren, for example, is a must.

Sanderson, Brandon. Rhythm of War (The Stormlight Archive) . Tom Doherty Associates. Kindle Edition. 

So it seems reasonable that trapping the Unmade required Radiants/Proto-Radiants that exhibited qualities that the Unmade would want to bond with; it's not actually just an ability Bondsmiths have, but Melishi could trap BAM because of their shared surges/fascinations/whatever actually drives nahel bond compatibility. Basically the spren is lured into trying to bond with someone, and gets sucked into the gem instead.

We know that the means of killing the Heralds forever was inspired by what happened to BAM:

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“We’ve used this metal for several Returns to drain Stormlight from Radiants,” Raboniel said. “It conducts Investiture, drawing it from a source and pulling it inward. We used it to fill gemstones, but didn’t realize until the fall of Ba-Ado-Mishram that capturing spren in gemstones was possible. It was then that one of us—She Who Dreams—realized it might be possible to trap a Herald’s soul in the same way.”

Sanderson, Brandon. Rhythm of War (The Stormlight Archive) . Tom Doherty Associates. Kindle Edition. 

It was weird why the singers would make Moash use the dagger to kill Jezrien; the only way that makes sense is if somehow Moash was the only person (that they could coerce/know about) who would be able to do this. Possibly something about Moash being effectively a failed Windrunner that might make him able to somehow bond with Jezrien's Cognitive Shadow.

There's WoB that seems to indicate that a Cognitive Shadow could be bonded to a person: Potential Mistborn Spoilers

If this is the case, then Mraize trying to get Shallan to use the dagger on Kalak takes an interesting turn. Could Shallan end up bonding Kalak instead? And what about Ishar?

Edit: Another interesting quote:

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There was a deeper mystery beyond that, a wisp of something she’d glimpsed while intertwined with Re-Shephir. It made Shallan wonder if this spren wasn’t merely trying to understand humankind, but rather searching for something it itself had lost.

Had this creature—in distant, distant time beyond memory—once been human?

Sanderson, Brandon. Oathbringer: Book Three of the Stormlight Archive (p. 325). Tom Doherty Associates. Kindle Edition. 

 

Edited November 28, 2020 by Seloun

[LordTheodore]

8 hours ago, Seloun said:

It was weird why the singers would make Moash use the dagger to kill Jezrien; the only way that makes sense is if somehow Moash was the only person (that they could coerce/know about) who would be able to do this. Possibly something about Moash being effectively a failed Windrunner that might make him able to somehow bond with Jezrien's Cognitive Shadow.

There is always the simplest explanation. The singers were afraid of Jezrien; afraid that he would fight back if one of them tried to get close enough to him to stab him.