(This display is not putting her especially at ease.)

The yin iron is a dangerous metal, fed resentful energies through human sacrifice.  All who have attempted to use it succumb to madness, the very nature of the metal impossible to purify.  Its existence in a place draws monsters and other forms of evil.  Hence why her piece has been sealed away in the Cold Pond Cave - a place of positive energy.  

Lan Yi herself had once been foolish enough to mess with it, though her friend Baoshan Sanren had warned her against it.  She had undone the seal which kept it hidden.  When she discovered that it was too unwieldy and uncontrollable to suppress, she attempted to seal it away again.  In this task, Lan Yi sacrificed everything and became bound here.  

Wow, it's really interesting how Lan Yi's friend warned her about dark magic being foolish to mess with and then was right. That's really interesting. I wonder what we can learn from this.

Wei Wuxian is not picking up on this message from Lan Zhan's glare.

If the Yin Metal can't be purified... has anyone tried using it to do good without purifying it first? Maybe if you pit up other resentful energies against it, to keep it in check somehow - 

No. That is a much worse idea. Someone did try that and it was horrible and he died horribly and everyone who loved him was extremely sad, or dead. So don't.

Yeah, but like. Maybe that guy wasn't as smart as Wei Wuxian. Wei Wuxian is pretty good at figuring things out.

"You are not smart enough for this. You are not smarter than Master Lan Yi. Be respectful."

"I didn't - no, I didn't mean it like that, Master Lan Yi, I swear! I think you're really smart, I - you're the cultivator who developed the Chord Assassination Technique, of course you're smarter than me! I really mean it!

But Lan Zhan - I'm not even going to try to use the Yin Iron, I never even said I wanted to do that. I just wanted to know if people had actually tried everything. It's theoretical! You Gusu Lans love theory, don't lie, that's all you have in your library."

He's already failing again. 

"Master Yan Li. As a descendant of Lan Clan, I swear to search for the other pieces of the Yin Iron and bring them here. I will not let anyone fall down dark paths."

"And I swear too, as the grand-disciple of Baoshan Sanren!"

"You are a grand-disciple of Baoshan Sanren?"

She looks wistful.  "I haven't received word from her in so long - since she chose to seclude herself.  She took on disciples?"

"In truth, my mother was Cangse Sanren. Baoshan Sanren was her master. I don't know very much about her, or my mother, but I want to honor them both and do what's right."

"I expect you to hold to your vows.  My grand-disciple.  Grand-disciple of Baoshan Sanren."

Lan Wangji, at least, seems to be taking the warning with the appropriate degree of caution.  And if they have been sent here, in this time, then perhaps they are in fact the best people who could be chosen.  

Lan Wangji does not share her confidence. But he is going to try to hold his vow, with everything he is, with everything he can do. He wants to hold the world together this time.

They both bow again, deeply. "Thank you, Grand Master."

She plays an elaborate melody on her guqin.  The sound echoes against the walls of the cave, and almost sounds as if it is playing in melody with itself.  The shard of yin iron appears from where it has been concealed.  

No longer needing to remain visible, she begins to fade back into the chill air of the cavern.  "Return all of the pieces, and return them to be bound."

"Master Lan Yi -!?"

"Quiet. Let her rest.

We will do as we vowed, Grand Master."

The moment Lan Yi disappears completely, the Yin Iron shard thuds heavily to the ground. 

He picks it up quickly before Wei Ying can try to reach for it. "We should go back. Time moves slowly in here. It is possible we are already missed."

"What about all these bunnies, Lan Zhan? Aren't they cold?"

"They are Master Lan Yi's company in her final days. We will come back for them later."

"- Ah? We will, Lan Zhan, you want to come pick up the bunnies together?" He giggles. "We can raise them between us, Lan Zhan, how cute!"

It - hurts. The last time Wei Ying talked like this, it meant something to him. The idea of a family together. It doesn't mean anything to him now. 

Wangji says nothing.

Wei Wuxian coos at the nearest bunny. The bunny ignores him. "Ah, we have such adorable children! Where are we going to keep them, your room or mine? I bet Shijie would like to see the bunnies..."

"Pets are forbidden at Cloud Recesses."

"Lan Zhan. You just said that we were going to raise the bunnies together! Now you tell me it's against the rules and act like I'm ridiculous for agreeing with you a moment ago? You can't keep playing games with my heart like this, Lan Wangji, the disappointment hurts too much."