After the events during Zhu Yan's rampage under the Blood Moon and Zhuo Yichen lost his family and clansmen he was too consumed by hatred and devastated by grief
He was preyed upon by Cheng Huang, but instead of turning him into one of his puppets, he turns him into a snake demon instead so he can fulfill his wish to avenge his family
Yunguang Sword rejects and denounces ZYC because he's no longer worthy, it even burns him if he tries to touch it
So he uses poison, his fangs, demon strength and his snakes to kill instead
On the other side, Pei Siheng still seeks CH for help and he is still slain by Pei Sijing, instead of bearing the guilt she goes mad and goes on a demon killing rampage
CH sees what a good killing machine she is and wants her to help him kill humans instead so he can harvest their souls to further his schemes.
CH doesn't tell her that PSH has become his puppet and can't die from a mortal wound so he can manipulate her
Tells her he knows a lonely young man who needs a big sister's protection and love, turns her into a snake demon as well
Introduces ZYC to PSJ "From this day forward, this is your jiejie. Be a good didi and show her our ways."
They become known as the snake siblings that terrorize the humans world, seducing humans with their beauty and tricking them with their ruthless cunning acts, then poisoning them
but they end up breaking away from CH and no longer works for him, truly embracing their demonic side
Edit: I put my softnspicytofu username because that's my account on AO3
