1, 3, 7-trimethylxanthine's Favorite

This was the full spoiler I found in Novelupdates. I had to search for it because of all the spoiler post.
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(Post from novelupdates)

I remember reading the novel way back, and there are alot of dirty witty jokes about the emperor’s stamina in bed etc. lol but the most memorable scene was when the Empress & Emperor finally confessed their feelings and was acting lovey dovey when suddenly there was a messenger saying concubine Lin had an illness and the Emperor had to leave...

Then just before the ML left for Lin’s place, the MC asked him to stay with her because she just had this feeling that something bad was going to happen  though the ML still left the empress to see Lin but Lin was all fine, just wanted to eat something together lol so the ML asked Lin did you fake the illness you never did this before etc. While they were still talking and taking their sweet time together a messenger came rushing and told the empress was kidnapped from her place by some assassins...

Then the emperor rushes to the MC’s palace, only to see everything in disarray and calls for a horse to go look for her while everyone was discouraging him saying it was dangerous. He goes after the empress and finally finds her but she is bleeding and he was crying and shaking her body really hard repeating “Empress wake up/say something...” and he loses MC to the assassins somehow again (? I don’t really remember the details) and he cries hard recalling the MC’s last words “can’t you stay with me because I have a bad feeling something bad is going to happen”

The MC was nowhere to be found for 2 years and during that time the ML turns into tyrannical king and everyone says it is due to the death of the empress. The ML finally fell in love with her, only to lose the MC so quickly afterwards so he was devastated. She comes back after 2 years with a new identity and in the end, they end up together.

(And this one was the one that was already posted here)

After the empress’ death, the MC goes back to her original body (she actually knew she had to leave the empress’ body that day but so she asked the emperor to stay with her which became a huge trauma to the ML) and after two years she goes back to the palace disguised as a male escort/bodyguard due to political shenanigans. The MC was actually summoned for a reason, and it involves this radical group founded by the Ryeokuk’s royals who wants to assasinate the emperor.

MC felt somewhat attached to the group though she enters the imperial palace and wants to protect the emperor seeing how the ML blames himself hard for everything and turned into someone completely different. The ML also left the empress’ place exactly as it was before, and gave out harsh punishments for everything involving the ‘late’ empress. The story kind of turns into a ‘BL’ romance as naturally the ML is reminded of his one true love-the empress from the MC’s expressions/habits when she starts working as the emperor’s escort and falls in love. ML admits his feeling for the new MC pretty early on, even though he is a ‘male’ escort/bodyguard, because he really regretted losing the empress without admitting his feelings. The ML even gets jealous saying you should be my man even in front of beautiful women lol and that causes Gong Lin to panic again and she tries the frame the MC, and even going as far as recreating the bloody ML’s trauma by harming herself and then copying the last words of the empress to hold onto the ML (which is pretty messed up imo).

MC felt angry about that incident and tried to tell him the truth, asking what if the empress is not dead but just hiding herself due to circumstances...but the ML reacted pretty voilently and storms out to go see his concubine. MC was disappointed because it reminded her of her last day - even when it is extremely important, he always chooses Gong Lin. So MC starts acting cold to the ML, refusing the snacks he orders for her, not meeting his eyes, and trying to avoid eating together etc. ML was scared that he hurt the MC, and he didn’t want to lose ‘him’ too so he asked why he was mad and he will try to fix whatever it is which surprised MC since he would have never done this before with the empress. ML even confesses that he is in an unrequited love with him in front of everyone, and tries his best.

That’s why when it was revealed MC is part of the infamous group that abducted the empress, it hurt him pretty bad as he thought she approached him on purpose and how he really fell in love with the person who killed the one he loved the most. While in prison, she tries to say she was trying to protect the emperor all along, but the ML was very conflicted with his guilt for the empress. Then when MC is alone with Hoon, she confesses her real identity considering her group obviously abandoned her and left her to die, and to prove it she recounts how they met the first time, with the intruder and the stone... The prison guard listening their conversation was member of the group, so the guard murders the MC while Hoon and the emperor was on their way to the prison. ML sees another bloody gory dead MC, a repeat of his worst nightmare, is shook to the core for the second time.

Fortunately, MC goes back to the empress’ body and a lot of political stuff about this prophecy about her and how she will rid of this group from Ryeokuk. She is given three choices including one where she goes back to her original world and forgets everything. But MC chooses to stay with the emperor and goes to the palace and meets the ML. He couldn’t believe his eyes and kept asking Hoon if he was seeing a ghost and is over the moon when the MC comes back in the empress’ body. Then she teases him saying I heard you were having an affair while I was gone and then kind of explains all the prophecy/politics to him. She couldn’t come back officially due to the threats from the group and the ML wanted to kill all the people involved in the group right away, but MC persuades him to wait to avoid shedding innocent blood and work to eliminate all the discrimination against Ryeokuk people which was the root of all this. The stubborn emperor couldn’t even believe himself that he actually listened to her and puts out an edict while she is sleeping. The book ends with the emperor kissing the sleeping MC saying I can endure everything for you (or something similar) with tears.

(These two post could be found on Novelupdates. Here's the link --> https://forum.novelupdates.com/threads/college-student-empress.105394/page-5#post-5927158)