This is something of a tough question. First you should be aware that MiHoYo's games draw heavily from Gnosticism for inspiration, and this is prevalent throughout
everything.
Second, both Genshin Impact and the Honkai games exist in the same continuity; the multiverse is a very real concept with wide reaching consequences in the games.
Third, there is a universal force present in every single universe, with no exception. This force will do its best to utterly annihilate Humanity and any other form of life that it did not create. This force is personified by the Will of Honkai, the closest thing to God—as the Demiurge—that exists in the Honkai-multiverse.
Now this universal force, or law if you prefer, can take many forms, varied and no less deadly for it. Plagues; Tsunamis; Earthquakes; Asteroid impacts; Supernovae; War; Societal collapse; Genocidal aliens; Radiation; Corruption; there is nearly absolutely nothing that the Will of Honkai cannot do with it's manipulation of Honkai energy in the material universe—save stripping you of your very soul. The most notable creations of the Will of Honkai are the Honkai creatures and the
Herrschers. These are the most prominently featured enemies in the games, of the various enamations of the Demiurge.
Now, to explain
why it hates Humanity would require some explanation of Gnosticism. For now, let's move past that and operate on the basis of the Will of Honkai's absolute hatred of Humanity.
Inspite of this overwhelming hatred, it has a twisted sense of 'fairness' in that it will appropriately scale the strength of a Honkai Impact to the technological prowess of the civilization it wants to annihilate; to give it a "fighting chance" of sorts. However, as the civilizations thrive and continue to march forward in their technological advancement, the Honkai Impacts scale exponentially in strength. This is when the Herrschers are born; corrupted humans, swayed by the sickly sweet facade of the Will of Hokai, accept a gift from the God during a time of emotional upheaval—a Herrscher Core. Merely upon
awakening, these Herrschers would kill millions of people. From then onward, they would be driven by genocidal impulses against Humanity. And they are truly fearsome foes; able to toy with the very laws of the material universe as the Will of Honkai had seen fit to grant them, and control endless legions of Honkai creatures.
Sovereigns in the truest sense.
However, the Will of Honkai has
also seen fit to allow a Herrscher to turn their back on it, should they wish, and assist Humanity against the Will of Honkai. All in the name of 'fairness.' And as the Herrschers are defeated, the next to appear grows yet stronger, and stronger still. Until, at last—the civilization has reached the greatest threat that any universe had managed to reach.
The Herrscher of the End; the 14th Herald of Demise.
God Kiana.
No universe in the untold and countless hundreds of billions of billions of universes in existence on the Imaginary Tree have managed to survive past this point.
None. And even should they somehow manage to do so,
it would never end. A never ending war fighting for your very existence against beings that would eventually make the 14th Herrscher seem like a paltry joke.
This 14th Herrscher is always some variation of Kiana, as we have seen so far in Honkai 3rd Impact; likely to be the Sustainer of Heavenly Principles (the Unknown Goddess) that we have seen in Genshin Impact. And it is this 14th Herrscher that destroyed the Twin's homeworld; a world advanced enough to travel through dimensions and visit foreign worlds, a world knowledgeable enough to attain immortality.
Is it not understandable then, that Celestia would destroy every advanced human civilization that threatened the extinction of all of Humanity on Teyvat?
It is also not understandable then, why Lumine and Aether would detest the gods of Teyvat and Celestia's systematic genocide of human civilization?
When this sword was last drawn, humankind was trying to preserve a doomed world.
That world was their last and only home.
This sword was drawn to defy that fate of destruction.
—But to draw steel against the law of universe that "all who exist must one day perish..."
Surely it must have seemed the height of folly.
But really, when one is faced when such a final fate, when up against the star-devouring darkness,
What weapon would be fitting for one to wield? It can only be a sword. If nothing else, it shall bear the memory of those who faced their ends without fear...
Sword of Descension
The Twins hardly have human morality anymore, I'm sure; just look at how easily Aether would've left everyone on Teyvat in return for reuniting with Lumine. And how could they, when they know that every single world they've ever visited has been scoured clean of life by the Honkai?
All they've ever had is each other and the perpetual threat of the Honkai.
This is presumably why Aether and Lumine (the twins) are also on a search for something, something they've been searching for billions of years (and that is no joke) since the destruction of their homeworld. They are quite possibly the oldest beings in existence on Teyvat, considering that the average lifespan of a star is 10
billion years.
Note how little the two of them pay any mind to the passage of time in the latest story quest.
These two characters have a long and lonely history.
In your long journey, you have seen the birth and death of stars as they passed you by, Scattering the darkness briefly before being consumed once more.
Wings of Descension
I personally believe it to be the existence of the Will of Honkai that they are searching for, but that is just pure speculation on my part. Well, that and the parallel the Twins hold to a component of Gnosticism knowns as æons, which the Twins come as syzygies—but that would take too long to explain and get too philosophical.
When you wield this sword, you search for the answers hidden within this world.
But because this search is perhaps why you descended to this realm in the first place.
After all, this one was forged for one such as yourself.
Someone who challenges and pursues.
Take it, walk this earth,
And cut open all the challenges and mysteries that it has to offer you.
Sword of Descension
You can see
this post for a brief timeline of the Twins at the start of the game and
this one for some info on Khaenri’ah.
I'm glad I at least watched it beforehand.
www.dualshockers.com
