The Founder of Diabolism - Season 1 Episode 15 Beautiful Tranquility

The fierce corpses only attack the Wen sect. Lan Wangji calls the technique sinister but deems the demonic cultivator a friend. The deceased Wang Lingjiao starts to attack Wen Chao and Wen ZhuLiu, and the cultivator shows himself. The cultivator overpowers Wen Zhuliu and uses use resentful energy to tear out his golden core, which he smashes. He finally reveals himself to be Wei Wuxian, catching the three by surprise. Wei Wuxian continues using his dark abilities to kill Wen Chao and Wen Zhuliu. With both dead, Lan Wangji warns Wei Wuxian of the price of pursuing demonic cultivation and fears he will not be able to control it. Wei Wuxian has lost his past cheerful self and cannot detect Lan Wangji's act of kindness when the latter offers to take him back to Gusu so he won't have to keep using the techniques. Lan Wangji leaves the battlefield in anguish as Wei Wuxian stays with Jiang Cheng. The brothers successfully take back Lotus Pier. To break the deadlock with the Wen, Wei Wuxian retrieves the iron sword from the Tortoise of Carnage's cave and forges it into the Stygian Tiger Seal.