I suddenly remembered this one arc that I like and want to re-read it again. Unfortunately I tried to search it but couldn't found which novel is it. It's a mermaid arc in a BL quick transmigration CN novel. The main character of the novel (which I don't remember the name) is a human and the male lead is the mermaid king. Meanwhile, the supposed protagonist of the world is the younger brother of the mermaid king (ML). For the background, before reaching a certain age, a mermaid can choose to become a female or male. While the females are sought by many people, the males was said to be very ugly although quite powerful. In this case, the male lead should be look ugly or that's what the MC thought. Long story short, the protagonist got lost in the human world and was picked up by his fated lover (?), which is the friend of our MC. Then the protag helps the MC's friend, researching in laboratory because the MC's friend has an illness and can only be cured with something related to mermaids. So protag sacrificed himself because he loves him. While the MC's friend likes him too but always prioritized his health first. Meanwhile, the protag's brother who is the mermaid king, went to the land trying to search for his younger brother and unknowingly encountered the MC. Some exchange happen and ML taken a liking on MC. Then they met again later on after the MC warned the friend to not 'torture' the body of the protag and trying to wake the protag to go back to the sea. MC brought ML to his house and saw ML going to heat. I forgot what happen next but ML then threatened MC to convince the protag to go home by putting a mermaid egg in MC's stomach?? So MC has to agreed and made a plan. At that time, the protag already has a rocky relationship with the MC's friend. After the plan was done, the protag got blackened I guess and went back to the sea and choose to become a male mermaid. Protag become from soft to the strong one. Then he kidnapped the MC and the friend along with the ML. While MC living leisurely with ML in the ML's palace, the MC's friend cannot accept it. Like the relationship and the positiom got reversed. Then MC and his friend tried to escape twice and it went successfully. But the MC's friend actually got pregnant and gave birth to a child. While the MC is still got in touched with the ML, the protag realized his mistakes. Getting some advices, then the protag tried to mend it by applying as a nanny for the baby. As time passed, the MC's friend realized it was the protag and then they got together again. I don't remember much but I guess that's the gist of it? I really hope I can re-read it again since it disturbed me sm. Oh and if I'm not mistaken, the MC is like a second rich generation. Thank you!