This is after Mo Xuanyu sacrifices his body to Wei Wuxian for revenge through ritual and passes out.It is all canon to The Untamed.

Wei Wuxian opens his eyes and he sits up while clenching his head in pain.Then he notices that he's in another body,not his.He stands up while stumbling a bit,but regains balance and looks around dumbfounded. 'weren't I dead..?How come I'm alive..and how did I get into this body!? Who's even is this body!?..' Wei Wuxian thinks in his head.

(You are Wei Wuxian)