Death March kara Hajimaru Isekai Kyusoukyoku (WN) - Book 17 - 17-1. Ceremony

Adventures of a reincarnated Old Man who died saving a kid who was about to get hit by a Truck. Reborn as the son of Duke DragonHeart, AshBorn being a unique individual with Dragonblood and elf blood in his veins he spends his childhood training with dragons and spirits of the world tree goes out into the world to adventure and climb the tower, traveling to different planets,learning the secrets of the universe as he fights an evil unknown enemy, in a perfect way of course

“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.”

Knowing she married a wrong guy, what would she do to deal with the subsequent marriage lives?

In order to pay for her brother’s medical expenses, she had to replace her own sister to marry the rich family's elder son . She always thought she was married to the elder one, but she did not know her husband was nominally the younger son He Yi Ning.

He Yi Ning, the real heir of the rich family, the ruthless business emperor, the ruthless workplace tyrant, and her sister’s dreaming husband. When she found out that the person who married her was He Yi Ning, she stunned and asked, “Shouldn’t i be your sister-in-law?” He smiled charmingly: “Are you unhappy with your husband?”