It's Actually Not Easy Wanting to be a Supporting Male Lead - Chapter 5.6

Lugis is an adventurer who accompanies the Hero’s team party. However, every day he has to fulfill tasks that are not pleasant at all. In addition, he is oppressed by the other members of the group. The only reason he remains on this journey is because of his childhood friend, Ariane. But she is fascinated with the team leader known as Hero. One day a shadow figure appears and tell Lugis, “Let me give you a chance. A chance to redraw your life.” And so, Lugis is given the opportunity to be sent to the past to restart his life.

However, he did not know that he was not raising just a harmless pet, but a biological weapon secretly made by the government… a real monster.

As a former librarian, Rand felt that he had been living an ordinary life, the one that he had reclaimed unconsciously every single day.

Recently, he has been a little upset because the pet fish he accidently brought home to raised seems to have undergo some strange changes

Well, it turned into a merman.

- Description from Novelupdates

Just before being summoned to another world, the protagonist’s summoning medium was secretly swapped by someone. As a result, he was not summoned as the “Holy King” but instead as the “Sex King.”The magic he can use includes water magic in the form of lotion, fire magic as candle wax, and his summoned beasts include a wooden horse used for punishment…Branded as “vulgar,” “useless,” and “filthy,” he is rejected, has his engagement annulled, and is ultimately exiled. However, the one who picks him up is a princess from a nation where the sex industry is legal.Using his modern knowledge, the protagonist begins rebuilding the nearly bankrupt sex kingdom. He introduces new fashion trends unknown to this medieval world and uses his authority as the Sex King to subjugate the girls. He then establishes modern sex service techniques.The protagonist's method of leveling up is through intimacy with his heroines (his concubine maids). The more intimate he gets, the stronger he becomes, and the wealthier the kingdom grows.However, the surrounding great powers will not stand idly by and allow a flourishing nation full of women. Thus, the flames of war are ignited.Utilizing his creativity, adaptability, and cunning, the protagonist repurposes his sex-related magic for military applications and overwhelms his enemies.As he gradually awakens to the path of conquest, he begins walking the road of a ruthless emperor. However, by his side is always a heroine, a celestial beauty who keeps his heart from straying too far…

He is a young man without a spirit root. It is believed that this denies him the chance at cultivation. However, a mysterious lady imparts a Supreme Demon Classic to him and from then on, he starts his path of cultivation.
He undergoes a total transformation to emerge as the most fearsome and powerful fiendish demon in his era, that even immortals and fiends are fearful of him, and saints are at his beck and call.
Mastering the Supreme Demon Classic is the turning point in his life. He is able to unleash his immense and divine power from within, illuminating the entire universe!

- Description from Novelupdates