[SPOILERS FOR THE DAINSLEIF QUEST]
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I think a lot of players are confused why Lumine is trying to help the Abyss Order, and, though Dain is the protector of Khaenriah, he disagree with Lumine

This can be explained if you understand the basis of HI3. Here's how

Some might think that Lumine is acting like a hero for trying to help the Abyss Order, but in fact, based on Teyvat 'Law', it's a stubborn move, and Dainsleif knows

During on one of our conversation, in Wolvendom (I didn't ss it sorry) Dain told us to be careful with Archons

This, and he told us that Khaenriah civilisation was very advance, until the Celestia destroyed it

Well, congratulations. THAT is one hint of the Teyvat Law, which is basically, "No civilisation must be kept roaming/stay alive; if it's too advance."

In Honkai, we already learned that there are many universes (or Bubble Universe) in a place called Sea of Quanta. At the center of the Sea, lies a giant Tree; Imaginary Tree, that created all these universes and the beginning of every human civilisation

However this Tree has a defense mechanism that will 'trigger' if humans on certain universe began to create advance tech and gain vast knowledge to the point they want unfold the secret of life, the secret of the Tree, that is by sending killer Honkai to destroy that universe

Probably Khaenriah managed to gain the techs and knowledge, and thought they'll be unstoppable. But Celestia knows. Celestia doesn't want Honkai to destroy Tevyat

And so Celestia gets their hand dirty to avoid that catastrophe, by crushing Khaenriah before Honkai come

Now that you know the basis of the Universe Law, let's move onto Lumine and Dainsleif case

Dainsleif, as a Bough Keeper, still hates the Archons but at the same time, he now know the Law, thus wants to keep the world as it is because HE KNOWS the danger

Meanwhile Lumine, who just got out of the 'Meteorite' still doesn't know the Law; or even if she knew, she wants to play as Hero; thinking that the Universe Law is stupid and destructive

So yes, tl:dr Lumine is selfish for siding with the Abyss while Dainsleif, though still haunted and feeling remorsed, wants to follow the Law and want to tells us that the fallen of Khaenriah as a lesson

Poor Dainsleif really love Lumine and sees Lumine as his light after Khaenriah got destroyed

He lost 2 important things on his life, but for the second time, now Dainsleif will not hesitate and do what it takes to get Lumine back

And yes, I sense death flag on Dain

One thing that I learn from miHoYo's game is every straight ship will sink/gone, reduced to atom

/Crying at MEI - Kevin, Owl - Ana/