This is really not mechanical ascension - Chapter 30 - "Hero

Cao Xin, a young man on earth, died in a car accident, traveled to a troubled world where supplies were scarce, and obtained a “Bronze Immortal Gate”…

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- Description from novelbuddy

“Does the vengeance of justice count as a sin?”Fushimi Shika, burdened with a system, gives his answer: “Yes! But I will take on all sins!”Avengers chant his name, leaving behind his name. Thus, the world gains a notorious serial killer.A grieving father avenges his lost daughter by killing the true perpetrator;A husband takes revenge for his wife through a locked-room murder;A child who lost both parents makes a desperate gamble...Fushimi Shika shields these unfortunate souls who have lost everything, leaving his unique proclamation at the scene:“This is divine punishment, I am the divine punishment”...Years later, atop Tokyo Tower, Minamoto Tamako points a gun at her partner.“Although I don’t know how you did it, you are the mastermind!”Amidst thunder and lightning, rain pouring down, Fushimi Shika raises his hands:“How rude, Rabbit Officer, I am a messenger of justice.”

“You are offering me a marriage of convenience, but princess… there's nothing convenient about me.”

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Izabelle had underestimated the fire she thought she was going to play with tonight. But how could she ever have foreseen that the man she had happened to come across with does not play with just a small fire, but a whole inferno?

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Excerpt:

“You going through this marriage with me might as well be like you digging your own grave. Because the moment you start to want more from me, I will divorce you. And the moment you break your promise and try to fight me… I will ruin you and break you apart completely. Mercilessly. You will regret ever meeting this devil tonight.” His threat was given in the same soft but cold voice.

But Elle did not even flinch. Her gaze never faltered either. This man was ruthless, and she knew that clearly. His eyes that moment promised nightmares and darkness without any promise of respite.

But no matter what he says now, it would still not change her decision. She truly had no other choice.

“I understand now…” she said quietly, steeling herself. “Since you don't trust that I will keep my word, how about we do this? You prepare the divorce documents beforehand now and I'll sign it. That way, once you decide to divorce me in the future, the papers would already have been signed and there would be no way I can trouble you about it. You would just need to send it to the lawyers and have it notarized.”

A heavy silence reigned before his quiet disbelieving laugh broke the silence.

“I'm at a loss for words, Princess Izabelle.” He looked wickedly amused. But then he started nodding in approval. “Fine, princess. I will marry you.”

Synopsis is in the beginning first chapter, so you can try to read it first.Zhang Fei is an ordinary human, but he is also an Incubus, and he also a Heavenly Fox.