Revenge of the Weapons (5.2 and Onwards)

[Theodric]

It only isn't right now because the emperor was assasinated and the chemical weapon the empire was preparing destroyed and now it's in a civil war, and could just as easily resume once someone consolidates power again. It's not like the empire had a change of heart. Zenos just threw a wrench into the gears, for his own selfish purposes even.

The use of a chemical weapon was to avoid excessive losses among a force that is incapable of using aether. The protagonists can paint it as a 'horrific' approach to warfare but conveniently it does no such thing when it comes to other traumatic methods of death such as blasting people with fireballs, running them through with swords, afflicting opponents with magical diseases or drowning/crushing them with water and debris as we saw with the fall of Doma Castle.

The use of Black Rose was a sound enough move and the real complication lay in the unintended side effects in the form of a Calamity. The weapon was also only used in response to Garlemald itself being invaded and in an alternative universe at that, so...

It's not even necessarily Varis who would opt to use it since apparently that universe still exists based on the Exarch's continued existence.

Either way, the entire plot felt rather contrived and messy. I wouldn't be surprised if it was wrapped up very quickly to justify diving right into Shadowbringers.