Reborn with an Old Enemy on the Day of our Marriage - Chapter 1 - In Spite of Everything, I'm Getting Married.

A series of short stories in the world of Toaru Majutsu no Index organized into chapters featuring Kanzaki Kaori as the protagonist, going around the world and space doing missions for the Church of Necessary Evil, however, there is more than meets the eye as each incident is connected.

One chapter was bundled with each limited edition release of the Toaru Kagaku no Railgun anime DVD and BDs, with all of them together being equivalent to a single normal Toaru Majutsu no Index volume

In a world of brooding heroes and dark justice, one mercenary stands out—not for his skill, but for his relentless sarcasm and love for chaos. A self-proclaimed anti-hero, he finds himself working with the Titans, much to their frustration. While undeniably skilled in combat and nearly impossible to kill, his real talent lies in driving his teammates insane with endless jokes, fourth-wall breaks, and questionable decision-making. Whether it’s swapping Robin’s gadgets with toy versions, turning serious battles into slapstick comedy, or giving monologues mid-fight, he thrives on pushing everyone’s patience. But beneath the humor, there’s a sharp mind, a tragic past, and a surprising willingness to do what others can’t. While the Titans may roll their eyes at him, they also know that when things get truly dire, he’s the wildcard they need. Love him or hate him—he’s here to stay, and he’s having way too much fun.

The dungeons appeared 3,000 years ago. New wealth and ways to gain power poured into the world, tipping the world’s power balance.

People rejoiced in their appearance. Now the poor could thrive, escaping poverty.

There were countless people trying to enter the dungeons. They formed a guild to manage them. We named them the Explorer’s guild.

All who ventured into those dark, evil places given a title to signify their status.

[Dungeon Scout]

A special occupation created to control the people who entered the dungeon.

A young orphan, adopted by dungeon knights and, would eventually follow in their footsteps.

His name was Vincent Schwartz, born in a fractured world of death and decay.

On the day of his 18th birthday, when children could ingest a special fluid and awaken.

Vincent awakened the unique [Summoner] class.

Able to summon distant beings that wielded endless power, able to perform terrifying attacks.

The only problem was the strange system, which appeared out of nowhere.

[The Demonic Choice System]

This system would give him random choices and give different rewards at different intervals in his life.

Would his system help him attain a wonderful life, filled with women he loves?

Perhaps the world will force him to fight, as the dungeon’s purpose remained hidden in shadows.

The simulation is over, and the entry is obtained: a showdown!

Looking at this entry, Zhao Meng was stunned.

“How should this term be used?”

“Hey! Here’s a wounded fox!”

…

A few years later, a woman who looked like a fairy appeared in front of him, and said:

“My lord, I’m here to repay my favor!”

This book is also called “Starting from Transforming Monsters”

- Description from novelbuddy