The exact order of events that brought the Demon Lord and her husband together is somewhat unclear in the Monster Girl Encyclopedia books. At any rate, after the defeat of the former Demon Lord, in a very unusual turn of events the remaining monsters (who were expected to largely cannibalize each other once robbed of their foul leader) instead rapidly acclaimed a new Demon Lord, a succubus. One of the Chief God'sheroes - who would generally be expected to deal with this development by lopping her head off - had been nurturing in his heart "doubts about why humans and monsters must continue to fight". On meeting the new Demon Lord, he stuck her with his pork sword rather than his steel one and they ended up married.
Incensed by the betrayal, and the Demon Lord's grand magic that turned all the monsters into monster girls, the Chief God descended from Heaven to annihilate her foe personally. Unfortunately for her, sex with a succubus is a positive feedback loop with regards to spirit and demonic energy, and the married couple had built up their power sufficiently that together they were able to wound and repel the Chief God. Although they won the battle, they didn't win the war; the present state of affairs is untenable in the long term. Hitting the Chief God over the head, or even conquering the world, won't bring the Demon Lord's plans to fruition; instead she needs to overcome the original Chief God's metaphysical laws that are preventing monster girls from bearing sons. For this, she needs yet more demonic energy power - and so her main strategy is fucking her husband all day every day in the Demon Lord's castle.
Being married to a physical god, and now possessing "extreme power even rivaling that of a god" in his own right, the Demon Lord's husband is not a typical incubus. "He deeply loves his wife, but he has a strong sense of justice, and a stubborn side to him. It is said that the minions get spooked from seeing how he fights with his wife" . When not fucking for the purposes of metaphysical revisionism, both he and his wife are accomplished Magic Gem crafters.
The Demon Lord's husband. Presently, he has been changed into an incubus by the Demon Lord. He was originally one of the heroes who came to slay the Demon Lord. He was a hero from right around the time on the verge of when the Demon Lord took over from the former Demon Lord. He had his doubts about why humans and monsters must continue to fight, and then he met the current Demon Lord. Together, they started having the ambition to fuse humans and monsters into one. He possesses extreme power even rivaling that of a god, and the spirit energy he possesses carries an equivalent amount of magical energy. By giving it to the Demon Lord, he is maintaining the present world. He deeply loves his wife, but he has a strong sense of justice, and a stubborn side to him. It is said that the minions get spooked from seeing how he fights with his wife. The world is in danger from a married couple's fight.
Naturally, the chief god was furious. It tried to eliminate the demon lord. It gave the power to slay the demon lord to a hero according to the system at the time, but since the hero betrayed the chief god and went on the side of the demon lord, the attempt at eliminating the demon lord ended in failure. The strongest of the monsters, the demon lord, and the strongest of humanity, the hero, joined hands to create a new world together. The demon lord who is a succubus, and the hero who was made an incubus by their hands, can increase each other's power infinitely by continuously having sex due to their nature. Their power has already reached the level of a god, and not even the chief god was able to destroy them. The chief god was gravely wounded in the battle, and as of the present, it can't move freely, so it's manipulating humans into doing its bidding through the church, giving many humans the power of a “hero,” and frequently sending extermination squads at the demon lord.
So all that is the truth of this world. It's already a myth level story. It doesn't feel real to me either. If The Order knew I had written something like this, I bet I'd probably be executed immediately. Whether or not this nonsense of mine is to be believed is up to you who have taken the book in hand.
On the same occasion, I was able to ask the demon lord herself about her future vision for this world that she's trying to change, so I'll summarize it on (p. 230).
For those of you who wonder whether this story is true, the demon lord and hero are both alive and well. Go visit the demon realm and ask them directly yourself. That is, if you're confident that you won't get caught by monsters along the way.