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I have read up to about chapter 1150, and it's pretty much been explained.
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Novel Spoilers
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Like the other poster said, Yuder has holes in his memories, so stuff might still be missing. He killed Kishiar simply because he thought he was doing the right thing. He served the Emperor, and the Emperor said he was a traitor. Yuder didn't want to believe it, so he went to Kishiar and practically begged him to explain himself. He told Yuder just to do what he needed to do. You find out that Kishair knew he was going to die soon due to his plate breaking, and so he set his assassination up on purpose, hoping that it would make the Emperor trust Yuder, making his life safe. Obviously, that doesn't work. The whole point is that in their first life, they loved each other but didn't trust each other, so they never communicated, which caused a lot of problems.
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Completely agree in this spoiler - also i just want to add
Spoilers
In the course of the novel, it was also revealed that Yudrein/Yudrain - 1st timeline name of Yuder - did not know a lot of this plus his soul was so damaged due to trauma which caused his memory to be full of holes so basically early chapters Yuder is somewhat of an unreliable narrator. And we as readers fills in the gap / holes along with Yuder. He got majority of the happenings correct however due to his personality he tends to ignore a lot of things which ultimately lead to his doom. His missed a lot of information regarding his colleagues, the politics and schemes etc. Kishiar the current timeline would actually help him a lot in recovering his memory and correctly interpreting the happenings.
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Spoilers
I've only read up to 750 myself. I am hoping with the emperor's awakening, he lives on for some many years with the empress. I hope the get an heir or lots of babies! And i hope Yuder and Kishiar gets at least a baby too! (But i hope it wont affect Yuder's abilities, i want him to remain a baddie). I dont care if my eyes burn reading to 3000+ chapters if it takes that long. LOL
Hello! I want to read the novel but I want to be spoiled on the reason why yudar killed the duke/ml in the past?