As the Minor Gay Love Rival in Het Novels - Chapter 5.7

Legend says, that there is a great demon, who is the human kind’s public enemy. He is the galaxy federations’ nightmare as he wanted to conquer the whole universe. On the first day, he completely took out the human fleet, plunging them into disorder. On the second day, he declared that he will wipe out the whole federation. On the third day, he declared——that the hero who shot him in their previous battle must marry him, or else, he will destroy everything.

The galaxy federation president and the council members had a long meeting after that. They decided to dispatch a soldier, Wu Xing Yun, and marries him off to the arrogant demon. Wu Xing Yun was once a hero who fought for the nebula, and henceforth, he was tasked—— for the sake human race and the universe’s peace, to eliminate mistresses, settle his matters with the mother-in-law, and fix the man’s crooked ways.

Wu Xing Yun was very perplexed when he asked his commanding officer: “Sir, why did it have to be this way?”

Commanding Officer: “Soldier, bear in mind, that this mission of yours, is for the sake of the universe and mankind’s happiness. You must hold on to your husband, and fight other mistresses! Do you have the confidence to accomplish this task?”

Wu Xing Yun, stood straight, saluted, and instinctively replied: “Sir, yes sir! I will accomplish this task!”

- Description from Novelupdates

It's simple. A vengeful wildfire of a woman. A tyrant who begs to be burned.When Aria crashes into Kael’s empire, hellbent on making him pay for taking her job away, she’s met not with defiance—but a dare. “You want revenge?” he taunts, voice dripping with dark invitation.“Then ruin me. Take your pound of flesh exactly where it hurts.” His proposition is a trap laced with sin: As expected he calls the shots in his bed, her contempt is his twisted aphrodisiac.“Hate me until your hands shake,” he growls. “But do it on top of me, where I can watch you savor every cut.”Their arrangement is a collision of dominance and delirium. Aria wields her fury like a blade, determined to break him. Kael? He thrives under her wrath, goading her darker, deeper, hungrier.But vengeance turns slippery when every command she hisses ignites his obedience, and every cold glare melts into bruising kisses. Soon, the bed becomes a battleground—her nails carving victory into his skin, his whispers a serpent’s promise: “Chain me with your rage, darling. I’ll still come back for more.”The rules are simple: No mercy. No surrender. But as their games spiral into something ravenous, who’s destroying whom? And when hatred burns this hot, what’s left when the ashes cool?

This is a prequel to the anime movie To Aru Majutsu no Kinsho Mokuroku – Endymion no Kiseki.

Just before the completion of the Academy City space elevator “Endymion”, the main characters Kamijou Touma and Index encounter a level 0 girl named Arisa. The Magicians are after Arisa, and they attack Academy City. A female leader named Shattoaura leads a unit from Academy City against the attack. Kamijou and Index learn from Stiyl that Arisa is key to a brewing war between the sides of Magic and Science. When science and magic cross paths, the story begins with Endymion as its stage!