(Spoilers for Stormlight Archives/Cosmere in general); it amuses me greatly that Retribution's One True Fear would be Harmony holding Nightblood. This, for multiple reasons, would probably never happen, but I like picturing it/his reaction to this scenario.
The joy I get from the certainty that Nightblood is inevitably going to be absolutely pivotal to the fate of the Cosmere is so real, you don’t even know. There’s going to be a whole space opera epic final trilogy and the most important and epic weapon is going to be this talking sword, I’m certain of it. It’s going to destroy evil sooo good.
To your actual ask: I think Autonomy would be even more scared of that than Retribution! Harmony seems to be her natural antithesis, Intent to Intent—cooperation vs individualism!—and the concept of merging with another Shard must be anathema to her, must inherently be a threat.
Also, I’m actually quite concerned above the reverse: I hope Szeth spends the next 10 years very carefully keeping Nightblood out of the hands of any agents of Retribution. Much though it would go so hard to have the Blackthorn riding a blood-red steed that is the Thrill and wielding a smoke-trailing, Investiture-devouring sword capable of slaying gods, I don’t, like, want it.
(That’s a lie. It’d be so fucking awesome. But only if Nightblood then is like, Wait, I’m not sure these people are evil? I don’t want to destroy people who aren’t evil. That would be…evil. Are you evil?!
@tanoraqui Counterpoint: There would be no more satisfying place for Nightblood to be then in the hands of an agent of Retribution/Autonomy if and when it it *independently decides* - thanks to all the character development it’s undergone - Hey, You People Are Evil!
And then reenacts Vasher’s favorite trick.