MASSIVE SPOILER HERE PLS DON'T READ IF U DON'T WANT TO.
Yes you're right about Sovieshit's plan however it will never happen because Trahsta's baby is not Souvieahit's, a man named 'Alan' banged her before she met Souvieshit, meanwhile Navier and ML will get married and they'll have twins this means that Souvieshit is actually the one who's infertille and his and Trahsta's supposed to be princess will later be proven that the real father is Alan, Trashta will later be punished through beaheading because of treason.
Yesss I can't wait for this moment to be on the manhua, we'll have a very long suffering til we get on the good part, I believe it'll be around 120+ chapters in manhua(160ch in novel) for them to divorce.
Rest assured that Souvieshit will definitely have a taste of his stupidity.
Also about Trashta's death, it is unlcear if she was indeed beheaded or if she drank a poisoin to suicide.
Trashta died, basically the empire is slowly getting destroyed ever since the trash became the empress, Souvieshit didn't want her to have the same training like Navier to be a proper empress because Souvishit likes Trashta's personality I think it's what charmed him in the first place because he once told Navier that he didn't want a business partner but a wife so he refused to train trashta which eventually destroyed the empire.
Trashta have a secret with her pregnancy because it's another man's baby and not Souvieshit so that person (I think the father of Alan which is the real father of trashta's baby) is using her as a cash dispenser, he would always ask for money and Trashta basically used the Empress funds for that as well as her typical greed. Naturally the empire suffers because of that and Souvieshit is the one cleaning after her mess without properly teaching her about her duties.
Later on Souvieshit realize all of his stupidity and breaks down, he suffered with mental illness idk the right term but during the day he have a mindset of 19 yearsold (this is the age where he was happy with Navier) then by night he return back to his old self suffering with regret and guilt.
Furthermore due to his infertility he don't have a heir and I think because he's guilty he decided to appoint Naviers father to take the throne after him until Navier's princess (one of the twins) be of age to take the throne.
So basically the prince will be the king of the western kingdom after Heinly and the princess will be the empress of the western empire (after navier's father)
Yassss gurl! I can't also wait because it's all fluff after they got married and the twins are sooooo ADORABLE!!
The prince takes after his father's appearance but with Naviers's personality, the princess looks like Navier but with Hainleys's personality. Hainly lowkey favors the princess because she looks a lot like her mum!!
ლ(´ڡ`ლ)
imma add one more
The bitch trashta used all navier's money for her wedding day. Her wedding dress was ridiculous. trashta punished her lady in waiting becoz she thought her maid would tell the emperor that she(trash) was the one who pluck the blue bird and not navier. she also command the guards to imprison her and cut her tongue.
one of the maid's brother was looking for her and doesnt know where her beloved sister was (btw his a reporter). until some maid told him the truth about it was trashta's fault.
one noble who caught that trash was using navy's money. he called the reporter to write trash wrong doing.
one morning trashta got up to bed and when she read the news she was too shocked to the end she gave birth. after that she was more miserable becoz sovieshit didn't love her anymore and stuff.
*
meanwhile in western empire navy and heinley are lovey dovey

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sovieshit's plan is: Divorce navier and make trashta Empress for a year before she gave birth and make the baby the heir of eastern empire. and after 1 year of trashta being empress Sovieshit would marry to navier again and make navier to take care for trashta's child.
pls excuse my poor English