So the story suddenly has a tone shift where Roth's (the demon) friend Balan ends up having the human girl he fell in love with at their wedding reveal that she was plotting to kill him in revenge for her sister, who died in love with balan after he wooed her and then left. The book hints that she falls in love with Balan while plotting her revenge, so when she gets the ability to control Balan she orders him to kill himself in front of Roth, who prevents it, and then she ends up dying a sad death where in pursuit of revenge she gave up her own happiness, while also somewhat succeeding since it impacted Balan heavily, although the degree of how much it affects him is a bit unclear. (We know he regrets it but its not clear if he regrets losing her or messing with her sister, ect.)

So the above is the first hint of tragedy, then this causes the main character to realize that demons could fall in love with humans, but she is worried that roth won't accept his feelings at first and kill her due to not wanting to come to terms with it, and only later regretting it once she is dead so she plots to return to the human realm. There is then a bunch of weird interactions where Roth is possessive and just overall gives a bad feeling when reading how they are interacting. She manages to succeed where she proceeds to learn that Abel ended up killing her best friend during his rampage, which roth concealed from her. This then leads to her being depressed, and Roth takes her back to the demon realm where we get more unhealthy behavior, where she wants to kill Abel in revenge for her friends death to where she even tries to use herself to roth fully in exchange for strength, which luckily he refuses. (by this I mean giving her body to him, in a s*xual way if that wasn't clear)

So overall this part honestly is hard to get through and to be honest there isn't too much payoff in my opinion. Like I said above she wants revenge and strength so Roth trains her, where we get some not stated but implied scenes where he trains her until she can't move and then washes her (unclear if he just bathes her, but it does imply that they don't have sex) which I guess somewhat feels like they are getting closer. After a bit roth takes her to the abyss where they run into Abel but he ends up apologizing and explaining that he goes berserk which then makes her relapse a bit since she lost her outlet for her friends death. Right after this she gets s**ked into the abyss where she sees illusions of her friends and family who are trying to get her to grow the abyss seed and throw the fruit into the abyss, which she doesn't do but this allows her to somewhat get closure.

Roth helps her escape but is in a coma because of it, and when she takes him back to his castle and runs into human 5-star mages. She ends up being kidnapped to the human realm where we get a bunch of annoying interactions of humans being humans with religion and indoctrination involved. Eventually Roth saves her but in the process he processes her (for literally no reason) where he learns of her origins... They then get together but we get a s*upid plotline of her wanting to sacrifice herself to the abyss since she believes it will stop the expansion (which would destroys both the human and demon world) which roth doesn't want her to do. Skipping over a lot of pointless details she literally just goes there, throws the fruit into the abyss, and its all solved. It ends with the two of them together and then they somehow make peace with the human world and they live happily ever after.
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Overall the ending felt very unsatisfying to me. The story takes a big tone shift which I think ends up feeling very bad as a reader since its unexpected. While being surprising is good in some contexts, here all it does is feel upsetting since what everything has been building too has turned into this train wreck where you have a hard time liking Roth. Its not a satisfying twist, its a twist that just feels bad to read.

Its then followed up by a truly shitty plot line. The whole abyss arc just makes no sense at all story wise. The conclusion is that the abyss becomes unstable and you need to occasionally throw the fruit into it to return energy and stabilize it, yet when she originally grew and got the fruit her illusions (which presumably are from the abyss because they want her to do this) are treating it like its some kind of taboo instead of just explaining the implications and asking her to do it. Then the illusions say Roth will stop her despite the outcome if not doing it is literal death.

Anyway, there is a lot lot lot more that I could go on about with this arc but this has already gotten long enough. Things that I think could of been done slightly differently to just make them feel better to read, things that are just poorly written in ways that are out of character, contradict other points in the story, ect. with just an overall very bad conclusion. I can tell the author wanted to have a big climax and emotional scenes but I think it just ended up hurting the quality of the book and just overall feels very bad to read.

Although I am saying this all about the ending being bad, I do still recommend this book. I truly enjoyed the first 80 or so chapters of this book and while I do think the ending fell flat it ended with I think a decent happy ending. Its very easy for me to pick apart a book after reading it with the negatives, but very hard to actually write out the positives of the book. While the ending did at times leave me feeling upset at the direction the story was taking, at the end I still enjoyed the story. Omitting the final abyss and friends death arc the story I think was good and is worth giving a shot.

I'm torn on this because I can completely understand where the people who hate this ending are coming from, since I agree with all of it, but at the same time I don't want to discourage people from trying this series. It's very easy in reviews to list out the negatives of a series, and its also easy when looking at a series reviews to see all the negatives and not want to read, but at least for me the reason why I end up with long lists of negatives in stories is because the rest of the series is good. Theres a lot that I like, which for me highlights the negatives and then to release the feelings that these negatives give me I complain in my reviews about them, but I don't want these complains to potentially turn someone away from these series since these long rants inevitably come from me caring enough about a story that the negatives upset me enough to rant about them. My overall point is I would recommend reading the story for yourself and making your own judgement. My preferences are different and my opinion is not right. While I can state how I feel its not what you will feel, so not just for this book but for any one on this website while reviews are a good thing to reference I would recommend reading the book for yourself.

tl;dr
The ending for the ML and FL is positive at the end.