Good.  That will give him a few days to consider whether there's anything more permanent that can be done about Meng Yao before he leaves.  And if he's very lucky, maybe Cloud Recesses will be tempting enough that he'll try to stay here and away from the Unclean Realm.  

In the meantime, Huaisang will wind his way back to the library.  It's a decent place to start writing, and maybe some of the other students are still there.  

Alas, the library is now empty.

Well, in that case... 

Carefully double-checking every corner just in case he makes his way to the suspicious shelf.  No time like the present, and he has the excuse of Wei Wuxian encouraging him to snoop in the library in particular if he's caught anyway.

Suspicious shelf: present, still suspicious. No lurking disciples are found in any corners.

Now how to open it... Pulling?  Perhaps checking for latches if it doesn't budge.  

Pulling will work if he uses some Magic Encouragement.

Once he tries that, the heavy section of self will swing slowly out towards him and reveal a dark, steep wooden staircase. The steps spiral down into a larger chamber below.

- shit.

Wangji hides himself behind one of the shelves furthest from the stairs and holds very still.

Huaisang can quickly scurry down the stairs (pulling the shelf closed behind him, in case anyone else comes into the library) then glance around for the secret music section.  

Those look like some music books in that shelf over there.

He glances around.  Steps cautiously in case there are any traps.  Tries to head towards the music shelf.  

There are no traps, but eventually his glancing will land on a surprised Lan Wangji, inexplicably pressed back behind a bookshelf.

Nie Huaisang might not actually be able to tell that Wangji is surprised, but he's definitely staring. That isn't a Lan disciple, as he assumed, that's Nie Huaisang. How would Nie Huaisang ever know how to get in here, not even Nie Mingjue would know how to get in here - 

Wangji does the sensible thing and draws his sword.

"Uh, hello Lan Wangji," he says, not masking the nervousness and complete confusion.  Why was it always swords.  He holds out his hands, which contain one paper fan and zero bladed weapons.  

Meekly, "I was looking for Wei-xiong.  I thought he was in the library.  He was when I last checked.  I, uh, thought that bookshelf looked like the sort of thing he'd been looking for?"

That was a terrible lie.  Nowhere near up to his standard.  Unfortunately, there weren't really any good lies to say.  

"It is not. Wei Ying has no idea what he is looking for." He approaches menacingly, which ideally distracts from how he was just definitely hiding behind a bookshelf. "The Room of Forbidden Books is closed to outsiders of the Lan Clan."

Also certain insiders. Such as Wangji. But Nie Huaisang is definitely even less allowed to be here.

Wangji jerks his head towards the music shelf, where it appeared Huaisang had been headed. "What are you looking for."

"I don't know.  I picked a random shelf - I don't know what's on any of them.  I don't know what you'd keep - is it porn?" he asks, trying to distract Lan Wangji.  He's pretty sure that is Forbidden here.

"Nie Huaisang explain yourself."

Why didn't he just wait and tell Wei Wuxian about the weird shelf?  That would have been way smarter.  Let him do the dangerous insane things.  While Huaisang stayed where it was safe.  Wei Wuxian had the skills to do something other than cower when there was a sword in his face.  Why didn't he realize that just because he was in the past before anyone had reason to be suspicious and he was just a kid, there were still People with Swords. Everywhere.

He thinks back.  What does he know about Lan Wangji. 

He deviated from the time path, before anything else.  Before there was a reason to.

Maybe...

How to ask in a way that's obvious if the answer is yes, but he can be worked into something else if the answer is no... Well, considering there's music.  

"Are you familiar with a flute called Chenqing?"

His sword hand trembles for a split second before he tightens his grip. Wangji lowers the blade, still staring. His knuckles turn white. "What do you know. How did we get here."

He doesn't stop acting completely, but he does switch to a slightly more mature tone.  

"I woke up this morning and I was here, in the past.  I don't know anything about how."

He pauses.  Explaining Everything About The Music is not a good idea right now.  Lan Wangji might be upset that he just tried to hook up his brother with the culprit.  

"As for being in the library.  My brother dies from Qi Deviation.  Maybe if I found the book with the Song of Clarity earlier, he would.  Have more time."

He dips his head in acknowledgement, sympathy. Wangji had heard; he was sorry to find out. Nie Mingjue had been a good man. - Is a good man.

"Song of Clarity can be taught. Stealing the music from here is riskier than learning from someone directly. Don't take anything from the room." If something goes missing, it will only become harder for Wangji to get in here when he needs to.

"Other goals?"

"Averting all of the," he waves his fan.  "Everything that went wrong?  That would be nice.  I don't know how, though."

Huaisang is overwhelmed, and looks even more overwhelmed.  There was just so much that went wrong.  Since it doesn't seem like he's about to be stabbed if he moves, he shuffles to a nearby solid object and leans against it.  

"Keeping Da-ge alive is the only thing I've thought of in the last few hours."

"Mm." He can certainly empathize.

Wangji sheathes his sword. "The war is coming. Only so much can be averted. Will you try to warn Qinghe?" 

If Huaisang were still in control of the Unclean Realm he'd be preparing them for a siege.  But he's not - hopefully he never would be.  

"I don't know.  We'd be more prepared.  Assuming I were to be believed.  On the other hand, Da-ge might choose to attack first and then the other clans would see us as the aggressors and it would stop everyone from working together right." 

Not to mention getting more of Wen Ruohan's anger directed towards them.  

"What do you think I should do?"

Nie Huaisang does not have a very impressive track record of doing things, in general. "Protect the Yueyang Chang Sect. Report that you overheard Wen Chao discussing a threat. Have them prepare for Xue Yang."

Wangji would like to be there when Xue Yang comes, to look for the Yin Iron if nothing else. He's not sure if he'll have time - there are so very many moving pieces, here. He can't return late to defend his home like he did last time.

"The Yin Iron shard in Gusu must be kept away from the Wens."

He nods in agreement.  "I'll send the warning about Yueyang."  That seems safe enough.