ok so what my limited intelligence tells me. During his time in a comatose state, which might not have been so comatose as he thinks that the original guy may have simply taken the memories of the years (?) he was in the demonic sect's hands, many things happened.
For one, there was a final meeting between prominent Sect Leaders, where the Hao clan (information ppl) basically exposed the Namgung clan's corruption. They were backed up by other clans that are insignificant for now. In response, the Namgung clan released Jiangshis onto them. These are corpse soldiers who seem to have been altered by the original white hair guy when he was younger as he essentially perfected them into fighting machines.
Of course, he was unaware of that but he was young and VERY naive as he's shown when the demonic Sect leader (who used to be his senior brother) reveal that their purpose was always to inflict violence at his behest.
Now this demonic blonde seems to have been brought back to life by the Namgung clan who must have been in cahoots with the demonic sect for some reason probably to do with power. We don't know any further nuances of the relationship between blonde and white-hair but we can assume they were close for white-hair to feel any betrayal. How close is a question I want answered in the future.
Now back to the Jianshi. The specific kind that he perfected needed some kind of wood that only grows on trees that grow in warm climates, the demonic blonde must have a power related to heat or light or something that allowed him to create an artificial environment that would allow them to grow in the dark cold place that the demonic sect must be situated in.
They've been actively at war with the murem (? fuck I'm forgetting how to spell things) for the time he's been imprisoned, his disciples were obviously helping the war efforts and it seems the number of jiangshi have started to reduce though 40 is still a substantial number of undead warriors. I can assume the og main character has also been fighting but it seems that despite him regressing he's fairly useless because why are we still at war? We shoukd be living happily ever after in the peaceful country plains or smt man, but no.
Now for what's happening in this chapter as far as I'm concerned. The demonic sect must have been formerly known as the Light sect, and obv the OG white hair guy burnt it all down and killed everyone after he killed his senior brother. We can assume it is something to do with him not agreeing with the way his developments in corpse soldier efficiency was used. He also this felt used and probably betrayed the blonde because of that.
Now he's come back and he wants the white hair guy back, idk if it's because he believes that he's the only one who could put up a decent fight against him or because there are some twisted romantic feelings there or just because he feels a sense of ownership over him.
Now the blonde has replaced the previous leader of the demonic sect by defeating him in battle and wishes to reestablish the Light Sect or at least the authority he wielded when he was the sect leader of the Light Sect.
One thing I will say I am confused abt is whether the rest of the murim was aware of the Light Sect as being a demonic one or if they considered it an Orthodox sect that was suddenly and cruelly destroyed.
But I don't think the translations or the visuals have much helped me to come to an answer about that just yet. I'm assuming they always knew it was evil and that the white hair guy was just too naive and stuck in a bubble to know that he was part of the 'evil' sect.

Can someone explain the lore to me cause am super lost
All the sect tea and what happened 200 years ago