The Dark Mage's Theory of Happiness

An adventuring party stops by a certain town. What should have been a brief moment of rest is suddenly turned upside down by a hypnotic trap set by a powerful demon lurking in the area.
Within a limited period of free action, you will witness a young mage girl whose common sense is gradually rewritten, as she slowly sinks into a mire of rising desire.
Not only the girl herself, but also the companions who stand close by, watching her… Yet behind the air of “helplessness” that surrounds them, could there be some hidden secret at play?
 

West Germany, 1981.

Eighteen-year-old Noah is haunted by the memory of his mother abandoning him at a very young age.
He lives with his father Leonardo, a failed writer and eccentric obsessed with researching “Dictator X.”

Though Noah wishes he would stop, Leonardo refuses to give up this strange pursuit, even as they struggle financially.

One day, an invitation arrives for an auction selling items connected to “Dictator X.”
Ignoring Noah’s protests, Leonardo sets off for the ancient castle where the auction is to be held, promising to tell Noah everything once he returns.

But Leonardo never comes home. With no way to contact him, Noah heads to the castle himself—only to witness something shocking.

Now trapped inside with the other guests, Noah is invited by the mysterious auctioneer to take part in the event himself.

In the peaceful Elf village, Aldos, the main character, bravely fought against the evil dark elf mage, Darius, to keep the land safe. But, after winning, Aldos found out a surprising thing – Darius’s daughter, Kora, really wanted revenge against him and his kin.
The king told Aldos to get rid of everyone related to Darius. Even though Aldos is a good person, he couldn’t hurt the young girl Kora. So, he chose to be kind and let her live. But, this decision started a chain of events that made Aldos regret to spare the young dark elf.

Heroines who seek revenge for various reasons make wishes to the demon race in order to gain power.
It is your task to assist them in their revenge while gradually corrupting their hearts.

One day, Tachio, an ordinary office worker, is suddenly transported to another world overrun with dark monsters known as Yamimon.
In this strange world, he is attacked by Yamimon, but is saved by the beautiful dark hunter Kurenai, with striking red hair and large breasts.
However, the moment he comes into contact with Kurenai,
he fuses with her special battle suit, Gear.
Thus, Tachio fights alongside her as her suit during battles,
while searching for a way to return to his original world (while being made to take care of everyday things at home).