The power I posses is the strongest in this world! The enemies I must defeat... are humans!?

I was Suddenly transported to another world. Being called the ‘Suitable One’, with the world’s strongest power “Holy Sword”. I thought I would be using that power to make a great impact in this new world. However, in this alternate world there are “inhumans”; Monsters that oppress the lands. Questioning my own convictions, I ask, is this correct? Who should I use this power for? This is the beginning of an Isekai battle fantasy!

Seven years ago, Yuto's older sister vanished through a mysterious door, leaving him with only a necklace. Now he has once again seen that same door, and watched two mysterious girls (Saya and Hina) go through it. Eager for clues about his sister's disappearance, he and his pet cat follow them through. However, he soon finds there's a reason his sister wouldn't let him come with her. In the deadly world on the other side of the door, his ignorance leads to tragedy. Now Saya insists that he travel through the door once again, to search for a way to retrieve the lives that he caused to be lost during his first journey.

Beings that create and control monsters and demons, beings that build and reign over malicious labyrinths, the chosen individuals to possess an overwhelming Unique Skill… They are the Demon Lords.

Demon Lords entice people to come to the labyrinth they’ve built and eat the people’s despair as their nourishment.

However, Creation Demon Lord Procell has decided that he will obtain people’s hopes and not their despair as nourishment, and built a happy town instead of a malicious labyrinth to collect human beings.

This is the story of an eccentric Demon Lord, with all its ins and outs.

During his 80th birthday, Watari Eiji has a heart attack. On his deathbed, he desperately wishes to fulfill his one wish in his next life. Instead of reincarnating, he spontaneously becomes his younger self.

From MangaHelpers:
Haruki is a teenage boy who has two friends that are not very ordinary. Ayana has been transformed to be part dog, as shown by her dog ears and tail. Ayana also has enormous lactating breasts. Airi, on the other hand, is a cyborg girl, and she's got tons of mechanical gadgets. The story begins when Ayana and Airi both want Haruki to join their club, and Haruki must pick one over the other. Ayana's science club has more to do with transforming living things, whereas Airi's has to do with more mechanical science. Their attempts to persuade Haruki to their side tend to be full of nudity and sexual arousal. Haruki is going to have a bizarre school life.