It's just an assumptions but,
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When I first read the prologue, it seems that yan'an was betrayed by the man in shadows (when the new empress's crowned) and he took the new empress side. That's why I didn't trust fei yue at first, cuz in many romance plot the ones that betrayed the MC was the ML (I assumed it's fei yue cuz he's the main consort).
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But then it seemed off,
In ch 1, yan an was dethroned and killed. the new empress has black hair and then said to yan an that "her lover betrayed her," and then the man in shadows appear then yan'an swear in tears that she'll never give her heart to anyone again.
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So, I assumed the transmigrator (current yan an) was transmigrated in yan' an past (before she was dethroned and killed), and the man in shadows was the white haired guy.
There's a few reasons:
1. In the prologue, fei yue said that "he only wanted to life in happiness with the MC and that he want to be by her side forever," so it doesn't seems right that he wanted her to be killed and to leave her side.
2. There's no Qian Han when the empress wanted to kill her (cuz the past yan'an didn't meet him). He'll definetely come to protect her.
3. The current yan'an before she transmigrated was already felt the heart break and already sweared that she wouldn't give her heart to any man, so it seems off that she sweared that again (when she was dethroned - ch 1) And when the past yan'an dethroned, her expressions was like it was her first time to be heart broken.
Then when the past yan'an dethroned, she's surprised by the man in shadow and she doesn't expect it to be him, while the past yan'an was always suspicious of fei yue. So he was another person. And cuz the past yan'an didnt meet butterfly and qian han, so that man was not them too.
4. Her dress. That dress (in ch 1) was more like the past yan'an style, cuz the current (transmigrator) yan'an changed her style.
5. How the current yan'an feels when she meet bai yu. It feels that her heart's hurt (?) and uneasy, is it really only because the past yan'an romantic feelings?
6. Bai yu dialogue in the prolog. You can compare it to another men dialogue that hinted their love for mc, but not bai yue, rather he hinted that only the person in the higher position would clearly see the background (?). Though that he said "before that, please dont die headlessly", but then what about "after" it happens ?
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So it's a possibility that the current (transmigrator) yan'an was transmigrated to the past yan'an (before she was dethroned) cause when she wake up, transmigrator yan'an was still in the body of third princess (her status's still a princess). And that man in shadow isn't fei yue (I believe).
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But all above was only guess and can't be proved, take it with a grain of salt..

But this's the true fact:
The new empress that want to kill yan'an in the future was the first princess, and not the 2nd princess (based on her hair color). So she's the TRUE VILLAINS not the 2nd princess, and this chapter proved it!
