I haven't read the novel yet but someonw commented this spoiler
Apparently, Ain (uke) was a servant of the Anatold's before (seme's family) and was raped by Seme's dad or something like that. He's basically pretending to be a servant right now to take revenge to the seme's family by taking their youngest son.
(I'll try to find a good translation so I can read the novel.)
Spoilers
- In the north, illegitimate children are thought to be a sin and are treated like they don't exist, very horrible while in the south they are not shunned but tolerated
- ML was born from a southern women who had relationship with the North lord believing them to be future husband and wife (not true). Since she's from the south she didn't abort the baby
- MC father bought him as sex slave because he believed to be someone without any existence and he would not be violating his oath of never having a mistress to his wife while she was pregnant with MC. (Hypocrite)
- MC brothers knew about him and treated him as nothing more than a disposable thing
- MC dad was raping him when he heard that MC mom gave birth to MC and just then ML decided how he would get revenge on every single one of the family members and decided that MC would become his.
- ML was shipped back to the north where he was treated horribly by everyone and rose to be rich to get revenge on his own family too for the death of his mom.
- ML was sexually abused by his step mom, family members and everyone
- MC doesn't know the sexual abuse by his father but hears a watered down version and thinks he will try to protect his "wife" properly
- ML changed his name to the MC's telling him because he is the wife but in actually because he will erase his family name and have been doing that slowly
It is a happy ending, by the way, from the MC's point of view. ML was going to kill him, but since MC was so sincere and emotionally resolute about being his husband, ML fell in love and decided not to kill him. They live their blissful married life, all the while ML slowly destroys and enacts revenge on everyone who hurt him. Funny enough, every time ML almost gets abused again, MC saves him, showing that he's serious about him. So it is a happy ending, ML manages to make MC his ideal partner in bed too LOL

So far I have heard that the bottom is crazy toxic and deranged BUT I have yet to see why??? He plans to kill the seme as revenge for being sexually exploited but that's not particularly crazy??!!!
So someone please give more details to the plot, I really don't mind major plot twists being spoilt