Duan Mu stabs Zhan Yan in the heart to get the antidote. But the antidote is fake. The poison will only go away if Wen Gu Wei Yu dies. The heaven kills the demon lord. Lord Jiang shows up to get Duan Mu to open up the abyss. Zhan Yan joins Duan Mu in the abyss. They return to 1000 years ago. (Source: jindreame at MyDramaList)