ok I basically just copied this from my reply to a comment three comments down from yours but here we gooooo. SPOILERRRRR
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So the royal (imperial?) doctor was giving the empress an examination and told her she was infertile. He told the mistress, the mistress confronted the empress about her being infertile and the empress is gonna be judged and ridiculed. But guess what our (un)lovable emperor is gonna do?(⌒▽⌒) His disrespectful dumbass asked her if it was true. He believed his mistress(who he knows doesn't love him) over the empress. Of course, our favorite empress goes off on him (as she should). Turns out the doctor lied so he could keep the empress on the throne instead of the mistress. He lied bc he knew the mistress wasn't gonna let the empress be if she heard she was fertile. I don't remember any other incidents but I'm verrrryyyyy sure there's more. Now this is the ending (there might be some parts missing in between because I really can't remember everything). Eventually the mistress gets executed for trying to assassinate the empress when the empress was on her way back to the palace from her family's home bc the empress knew about her whole plan and hired her own assassin to frame the mistress (well she did do it but you know the mistress doesn't leave much evidence behind). Apparently, the mistress tried to assassinate the empress's sister the same way in the past and that's how she knew. The mistress gets locked up and the emperor knows what the empress did to get her in there, but does nothing because he knows she really tried to kill her and she deserves it. After the mistress gets executed, the empress wanted to leave the palace and live peacefully but the emperor begs her not to go. She really can't go anywhere unless she commits a heinous crime so the empress stayed. In the end tho, the emperor finally listened to the empress and changed for the better because he fell completely in love with the empress (he just has a lil crush on her right now). The emperor and the empress did end up having kids(that's what I heard).
And this is what somebody told me directly about the dialogue between the emperor and the empress about her so-called infertility(this is word for word so if there's typos I'm sorry):
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That trashy emperor and our MC fight with each other about her infertile, she told I'm not infertile and i never checked myself so how you supposed to know about this? But later she noticed actually he did without her permission.. that red head bitch checked her fertile and emperor just let her do that.. so empress noticed that.. and empress told "so what's the difference between you and she(ex empress)? Both of you just same for me" so his mind totally broken by that words so now he just ran after her to everywhere while for a little attention from her or for a word from her but she pretend like he isn't even exist puhahshs so funny.
Like father like son ...they both took a mistress neglected their queens drove them into madness....but now in this life time I hope that the king takes care of our queen ..and not to repeat the same mistakes ... (● ̄(エ) ̄●)