
After MC died, the system told him ML gained too much darkness points that it was causing the world to be unstable and potentially make the world collapsed. So they needed the MC to go back and fix the problem, since it was also MC's fault this happened (And lack of manpower).
The system even threaten him if MC doesn't do it, it will be deemed mission unsuccessful, even though he did reach the goal of hitting the necessary darkness point. At the point they were telling MC the news, 9 months had passed and ML had actually killed all but 1 of the emperor's children. (Which is the emperor that MC is possessing)
His new mission in the emperor's body is supposed to matchmake the ML's love interest in the original novel to the ML. Because the love interest in the original novel is suppose to save ML from the path of darkness.

Oh I thought I translated wrong. I actually didn't. The system's supervisor did say ML killed all but one of the current emperor's child. (The current emperor is the ML's uncle)
BUT ML and new emperor aren't related. ML killed all of the current emperor's sons. Since the current emperor had no more sons left to inherit the throne, ML forced him to adopt a child from the branch family. His name was Wei Yan and changed to Qi Yan. (Since the current emperor adopted him) So this adopted child was the only one that was not killed and made to become the new emperor. MC then reincarnated into the new emperor.

He did not betrayed ML. He accidentally ate a poisoned snack that was meant for ML.
In the original novel's plot line, a servant (Acting for the current emperor) was going to serve a plate of poisoned snack during ML's meeting. So that ML will eat the poisoned snack. But a random court officer ate it and get poisoned instead. (Since ML doesn't eat sweet snack)
However, what actually happened was that when the servant passed by the library MC was in, MC wanted to eat the snack. So Xu FuQuan stopped the servant and said to serve it to MC instead since ML doesn't eat them anyway and just get another plate that is less sweet to serve it to the meeting. So rather than the random court officer, it became MC who ate the poisoned snack instead.
After the poisoning event, the system then informed MC that this is actually part of the original novel's plot, just that the target just somehow changed to MC instead.
And because of this poisoning event, the system informed MC that ML's darkness point moved and MC has completed his mission.
This poison actually doesn't kill a person immediately. In the original novel, the court officer took 7 days to die. MC asked the system if he can leave now since he completed his mission. The system said it would be too suspicious if MC died too quickly. So they delayed MC's death by 2 days.
Some stuff happened during the 2 days but at the end, ML ran off to get some food from the New Year's party for MC and left MC alone. The system then asked if MC wants to eject from the world now. MC says to wait a little because he wants to return back ML's jade pendant. (Left it on an obvious spot at the head of the bed) After that, MC asked the system to eject him. And then MC died.
Can’t wait till he dies. Like I mean that in the sweetest most nicest way possible I CANT wait till he fakes his death