Highschool academy battle harem where the setting centers around an isolated schooltown for elite technomagic warriors of the future that's mostly populated by women.
One day MC causes a stir by enrolling into the place as one of the rare males capable of wielding their technomagic at combat proficiency and having the top girl of his grade challenge him to a duel that seems overwhelmingly in her favor until he literally cuts through her magical inferno using a rare unique ability he calls "Void Cancel" and demonstrates that he is at least her equal in battle. Hijinxes follow that pushes the two of them together and pull a few more colorful girls their age into a free for all for the MC's attention.
Things go as swimmingly as one might expect as they fend off the threats from a shadowy organization dedicated to reviving the ancient weapons left by the ancient extinct demon race wiped out by the legendary hero of light. The dead heat between the girls even reaches the point where it's really between the first girl and obviously best blond girl.
But then one confrontation changes everything where it's revealed that the organization wasn't trying to revive the old demon lord but one of the saints that helped the hero slay him and have her ascend as a goddess. This sets MC off and he takes out the entire facility they were fighting in using powers beyond anything he should be capable of.
The villains are defeated for now but MC is still unsettled by the events and asks for some time alone away from the school. Everyone lets him do his thing except for the blond girl who secretly follows him to make sure he's okay. MC catches her soon enough but allows her to accompany him to the ruins of a far away demon fortress. Once there he begins prepares an unknown magic ritual and has the girl take center stage for it.
He reveals to her that his power isn't really magic cancellation but displacing things in time and space and that the reason he freaked out that day was because up till then he was sure the organization would never succeed at reviving the demon lord. The reason why is that he himself is the demon lord in question, or least the last fragment of him left. Centuries ago, he and the hero struck mortal blows against each other in the final battle but unwilling to die, he cast a spell that ripped the essences out of their bodies and froze them all in his special dimension until they all finally stabilized sixteen years prior. When that happened, he returned back to the world using the hero's body and altered their essences to contain demonic elements and seek out new vessels for their souls.
Since then, he put himself to sleep to revitalize his new body and to allow the former hero party to mature in this new world of theirs so he could reunite with them as his vassals for another attempt at world conquest. He intended to allow for more time to grow their loyalty to him but since he learnt of the organization's true end goal, he had no choice but to speed things up since the advances humans made already make his goal difficult enough.
That's where the ritual comes in. The blond girl is the saint they intended to revive, the first girl is the former hero and the rest of the harem are those who got caught in his spell that day. Despite his machinations, MC does feel something for them and his sentiments towards them makes him give the girl a chance to fight back. If she manages to defeat him in a duel he'll let her end him then and there but if he wins, he'll execute the ritual and forcefully awaken her demon elements and enslave her to his will.
She's conflicted at first and fails to steady her hand on her sword but then she slowly walks up to him and pushes him out of the magic circle he prepared. Taken aback by this, he prepares to fight her but watches in shock as the girl activates the ritual herself. With tears in her eyes and a gentle smile on her face, she tells him she'll follow him to the end of the world and lets herself be turned into a demon. The ritual proves to be a flashy and painful process and when it finishes he rushes to her side out of fear more than anything. Sporting new demonic traits that ripped right through her clothes, the girl stays unresponsive for brief spell but then life comes back into her eyes. Still seemingly dazed she wraps her arms around his neck and hungrily asks him what they should do now.
The actual focus from there is the best girl conquering his route bit by bit from that point on as his unofficial wife and that MC's corruption h-plot driven scheme leads to the other girls rising up as the new feudal lords of the era because he didn't account for the ritual and organization tech granting them the ambition and power to break free of his mind control. The finale would have had MC and the ex-Hero going through the ritual themselves boost their powers in the final battle only for MC to abandon his plan when ex-Hero pushes him to choose best girl over his ambitions by declaring she can't let herself as a hero be there for MC and that her faction will definitely kill his true love if he doesn't stop her from making her final stand for him. It ends with MC and best girl displacing themselves to the future to live out a comfy life together as wandering travelers to see if the other girls reformed the world like they wanted to.