Fate/Ultimate Antagonist

Getting Isekai'd to another world is indeed awesome, especially embarking on an adventure as a hero to defeat the Demon King, who turns out to be a woman, and making her part of your harem. It is truly the dream of millions of people hoping to be Isekai'd to another world.

If Reinhard were to get Isekai'd like that, he might smile for the rest of his life. However, fate has other plans for him, as he becomes the unfortunate isekai victim, landing in the rabbit hole known as the moonlit world. With numerous psychopaths roaming around to fulfill their dreams, he must strive to defend his small life in this chaotic world, especially when the world is set to last only another decade.

Fortunately, luck from nine generations of his family comes to his aid in critical moments. Finally, he gets help to secure his small life:

[Ding!]

"Eh? Has my help finally arrived?"

[Congratulations, Host! You have been chosen as the host of the Antagonist Simulator System!]

"Finally! Fina-... wait... What do you mean by Antagonist? Shouldn't it just be a Simulator without an Antagonist?"

[As the name suggests, the Antagonist Simulator System is specifically designed for individuals with high aspirations to become antagonists, and you have been selected to be the Ultimate Antagonist! Congratulations, Host!]

"Huh? Ultimate Antagonist? Wtf? How does someone as gentle as me fit as the ultimate antagonist?"

[Not so.]

[Host, you undoubtedly possess a strong motivation, clear plans, firm will, undisclosed secrets, outstanding execution, remarkable personality charm, and the determination to subvert the world.]

[Undoubtedly, you are the perfect candidate for the ultimate antagonist.]

WTF?

...
[You were borns as the son of the King Vortigern.]

[You pulled out the sword from the stone but inserted it back, and the sword emitted a resounding mournful cry]

[You promoted agriculture, popularized education, abolished nobility, knights scorned you as the ominous son of the disaster-bringing white dragon]

[You sacrificed Vortigern, gathered the remaining flames of the remnant age of gods, and launched the final assault towards the inner sea of the planet]

[Your wife and daughter died at the hands of Ares]
.
.
.
[You set the Olympus Temple ablaze]

"Oh? Looks like my adventure won't be boring."