Wei Wuxian vanished, something woke up the Xuanwu of Slaughter, and then a bunch of fierce corpses started attacking.  

"Shit.  Wei Wuxian is from the future."  The corrupted, dangerous Wei Wuxian.  

"Is that bad?" 

"Yes."

"Wei Wuxian vanished for three months, a bit after the takeover of Lotus Pier.  When he returned, he had learned Demonic Cultivation and the ability to raise and control armies of fierce corpses.  At first, there weren't many mental effects.  The only unusual thing was that he refused to even carry his sword from that point onward, instead taking a strange black flute everywhere.  

"His behavior became less stable over time.  He left Lotus Pier to live with a group of rescued Wen prisoners inside of the Yiling Burial Mound.  A-Jie marries Jin Zixuan, and they have a child together.  He is invited to the one-month celebration, despite having alienated every other sect.  While on the way, he kills Jin Zixuan and dozens of other Jin along with him.  When all of the sects get together to discuss what can be done about his growing instability, he attacks them all.  I was there, and A-Jie.  She is injured by the corpses, and dies on the battlefield.

"I was the one who had to kill him, in the end."  With A-Jie gone, no one alive would be able to talk him down.  Leaving him would mean more massacres in the future.  How many lives would he have taken from that point onward, if he'd been left alive.  Hundreds?  Thousands?  

"If he's from the point he died, he's not my brother any more.  I don't know what he is."  

"I see. I'm so sorry." 

He takes a moment to pull himself together.

"The Yiling Patriarch will be wherever the fierce corpses are."

He's hoping that Lan Wangji isn't one of those corpses.  Not that it wouldn't serve him right, but it also wouldn't do anyone any good.

"I don't know where that is more specifically than Qishan." 

"I suppose I'll go looking.  Tomorrow, preferably.  Could I... do I still have a room here?"

Nod. "You'll need an explanation for why you're back empty-handed and without Wangji, but..."

"I woke up in my old body last time, so it probably happened again this time," he considers.  He checks his robe for any letters or information that had been tucked away.  Nothing.  "I don't know why he returned.  Perhaps he lost their trail?"

He is trying hard to ignore his memories of the Yiling Patriarch's crazed crying laughter at the end, before he set his Demonic constructs on his own sister.  Had Jiang Cheng been running away from that?

"Perhaps. It will do." 

He stands, bows politely, extracts himself from the office, and goes to his room.

There are differences from how he last left his own room, but it does look like a place where he's been staying. Apparently the other Jiang Cheng didn't pack his things before leaving. 

It's early even by Cloud Recesses standards, but he is going to just fall asleep now.  

The next morning dawns as bright and peaceful as if the sun rose on a world that didn't contain the Yiling Patriarch. 

He might as well go back out before anyone thinks to question why he came back.

Without a specific goal, he'll just travel in the general direction of Qishan.  He'll be able to get better rumors out there.

There are so many rumors! Also fire-damaged Wen outposts. The rumor mill isn't entirely consistent or, one assumes, accurate, but judging by the general threads Wen Chao and Wen Zhuliu are also dead. 

As he travels further in he can learn that apparently Wen Ruohan is also also dead, very recently, in his own palace at Nightless City, and people are praying that the Xuanwu's curse will be satisfied with that. 

One can only hope as much.  

He travels towards the most recent attacks, trying to figure out the pattern of where they'll go next.

The most recent attack was Nightless City; before that it seems to have been nigh-random zig-zagging. 

That seems like a place he might stay for a day or two.  He'll go there, and keep an eye out for any especially close Wen outposts which hadn't been touched.  

Hey is that an oddly-familiar figure washing a pot in a stream?

...What has the Yiling Patriarch been doing?

He ducks behind a tree and watches the doppelganger.  

Washing a pot. Not looking particularly worried about anything. The pot appears to have been used to make congee. 

He breaks cover and approaches, hand ready to pull Sandu from its sheath if necessary.  

"Who are you?"

"What the fuck? I'm Jiang Cheng, who are you?" 

His hand goes to his own sword, which--if he's some kind of forgery constructed by the Yiling Patriarch, he's a very good one, because that sure looks like Sandu. 

"I'm Jiang Cheng."  The real one.

Keeping his distance, he looks at the other.  In particular, he checks for any signs of undeath.

"Is Wei Wuxian creating copies of people now?"