Jiang Cheng storms away from the courtyard, avoiding his siblings, heading somewhere both private and hopefully far away from Lan Wangji.  Of all of the people who could have been mysteriously sent into the past with him, Lan Wangji was... not the worst, but there was room for improvement.  Jiang Cheng, Lan Wangji, and Wen Ning had all woken up 8 years in the past at the start of their education in Cloud Recesses, and now had to deal with each other in addition to trying to fix everything that had gone wrong the first time.

His hand still gripped Sandu's sheath tight enough that the texture of the metal was going to leave marks on his skin.  Lan Wangji had attacked him, thinking that Jiang Cheng had been trying to kill Wei Wuxian.  

This is sufficiently distracting that he doesn't notice the unusual sound of a large reptile slithering its way through the covered walkways of Cloud Recesses until it's too late.  He turns around and sees a mirror rushing towards him.  Jiang Cheng pulls out his sword.  The mirror is shoved into him, and he falls through it.