mfa

To make it short, after the prev arc, Ryeong's souls have almost disappeared or shatter (they say that humans have 3, but Ryeong is hanging on a broken single soul rn), so the old lady they encountered on their way (who Ryeong helped and they considered each other mother-son) told the group they needed to find a stone made from the soil that absorbed a thousand people's blood.
They solved a riddle and got transported to this weird soul trap made by a man who can manipulate souls. This man was a kid who the woman from before tried to save (and he's the reason she died) and later on was taken by Ryeong along his siblings to live in the island he used as home at the start of the series (actually at some points of the story we see him recoil at the word father and remembering the kids, so yeah, it was these kids). He was trained by Ryeong, but he realised he would have an ugly death if he kept following the way of cultivatoon and all that, along the fact he had a nasty manipulative personality, so he blocked his qi and stopped training him. At some point, Ryeing told the kids to return to the human world and the kid overheard some ascendants who said they needed to kill him, cause they knew he was related to Ryeong, and mentioned the qi being blocked. The child escaped and never returned cause he felt wronged by Ryeong, and then lured him to a trap by lying.
What we see in this last ep (158) is that Ryeong always knew the man was kinda twisted and also that he does have the souls he needs to extend his life just a little more, so they gotta find a way to get those in the next chapters (idk more cause i refuse to read the ai translations)

This is hella long for a "short" explanation despite lacking so many details lmao my apologies but hope it kinda helps