Disclaimer: I don't own How NOT to Summon a Demon Lord. It's written and illustrated in the real world by Yukiya Murasaki and Takahiro Tsurusaki in that order.
A Demon Lord's Hostile Takeover
Fifth Month of Year 164, Starfall Tower, Kingdom of Lyferia
'Can't believe I'd be doing this.' Medios thought to herself as the Summoning ritual's preparations were being completed.
Having been raised and trained by the same Mage who also took in Celestine Baudelaire of the Mage's Association of Faltra City, Medios, while not without potential, hadn't quite caught up to Celestine's exceptional talent in almost everything magical. Her one accomplishment was that she mastered Aura Vision, the ability to see the flow of magical power, three days before her sister figure was able to. Therefore, upon growing to adulthood, she became head of the Beloved Traveler's Hall in the slave trade, which was a profitable market, if controversial by her world's standards. Of course, under her watch, the slaves Medios would sell were meant to be kept physically and mentally healthy as well as treated as if they were children in an orphanage meant to go to good homes, which kept both her profit margin and her good graces in Celestine's eyes.
Nonetheless, managing such a business required good security for herself, her wealth, and the "merchandise" in her care, but Medios was reluctant to hire a trained bodyguard out of fear of betrayal, which was when Celestine suggested she should get a Summon for protection. After thinking it over for some time, Medios agreed with the idea and came to Starfall Tower.
When the time came to do the ritual, Medios poured in her magical power and summoned a creature that she honestly wasn't suspecting. Unlike other Summons that ranged from flying spies and fire-breathing monsters, the then unconscious being in front of her took the form of an attractive man in black clothing that even without the fake horns, had the aura of what could only be a Demon and a very powerful one at that.
'Well, I practically asked a powerful one and he is good looking.' Medios shrugged to herself with a smile before taking out the magic stone required for the Enslavement ritual and began the necessary chant. "I hereby command you in the name of Medios. You shall answer my call, come mine alone, and forever obey me." She then placed the stone in her mouth and kissed the Demon on his lips to seal the pact.
It was then that the Demon opened his eyes, became aware of his surroundings, and stepped back on the floor in shock declaring. "What the Hell?!"
A little surprised by his reaction, but hiding it well, Medios turned to him with an amused smile saying. "No need to be shy, dear. I admit I was surprised to summon a Demon myself, but since I used the Enslavement ritual, you're bound to me and me alone."
As the Demon seemed to struggle what to say, Medios looked at him in confusion before deciding that being kind wasn't bad on its own. Stepping down on one knee, she then said. "Why don't we start over friendlier? My name is Medios. I am the slave trader in charge of the Beloved Traveler's Hall, and I summoned you to protect both me and the slaves in my care."
The Demon then looked at her warily and found his voice. "A slave trader?"
Seeing his suspicious glare, Medios placated him with her hands and explained. "I know how it sounds, but I'm not like the typical slave trader as I will show you when we go to my tent."
After a moment of internal debate, the Demon then looked at her frozenly. "You have some nerve enslaving Diablo the Demon Lord."
Surprised, if somewhat skeptical, Medios replied in question. "A Demon Lord?"
Turning his hands into fists for emphasis, Diablo then went on to say. "If you value your life, you will make it up to me by kissing me as you did before once more."
"Really, now?" Medios teased with a flirtatious grin before a green magic enveloped and moved her body to do the deed against her will. 'What the Hell?!' She thought in shock as she kissed him for the second time that day before the magic enveloped around her neck to form a slave collar which surprised both her and Diablo even more so. It was then that Medios couldn't decide what was more ironic. That she mastered Aura Vision, but didn't see that Diablo had the power to reflect all magic, or that she was a slave trader now enslaved to him. Only one thing was for certain. 'Celes is never going to let me live this down!'
By the looks of things, I'm the first person to write about the visual novel that recently became an anime, How NOT to Summon a Demon Lord. Hooray for me! But on a more serious note, I think you all can figure this to be like one of those stories that have Bell Cranel from DanMachi or Natsu Dragneel from Fairy Tail in a different Familia/Guild from canon. I honestly don't know how this would change Diablo's other worldly adventures, but I think it would be interesting.
I chose the name as a reference to the corporate tactic of taking over other companies, like how Diablo took over Medios' slave market by accidentally enslaving her. It's honestly sad that there aren't any "good slavers" like her in much fiction as I'm sure my fellow One Piece fans can agree with, but that's beside the point. I also started with just one character instead of adding Celes to make it an easy start and because while I think Edelgard and Alicia would be an interesting substitute for Rem and Shera, I haven't exactly figured out if they have magic and that one has to have magic to be a Summoner in the world of "Isekai Maou" as some call it. Anyway, that's about it.