Breaking Into Another World: I Will Save My Sister!

[ I'm in WSA 2024. Please vote for me!!! ]
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Jack Saunders and Anna Saunders were just ordinary siblings who shared a love for Japanese culture and anime. However, their visit to the bustling streets of Akihabara in Japan took a dark turn when a mysterious magic circle appeared, trapping Anna inside. Desperate to save her, Jack tried everything he could to break through the barrier of the spell, but he was unsuccessful. He could only watch in anguish as the summoning spell whisked his dear sister away.

But this was not the end. When Jack discovered that multiple people were being summoned at fixed intervals, he realized that if he could break the barrier of the summoning spell, he could transport himself to the same world and save Anna. Despite the seemingly impossible task and the toll it took on his sanity, Jack dedicated himself to relentless study and research. He delved into every scientific subject imaginable, seeking a way to predict and break the summoning circle.

Two decades later, Jack succeeded. Armed with modern technology, full body augmentation, and the most advanced AI implanted in his brain, he finally broke into the fantasy world.

But the challenges awaiting him were far greater than he anticipated. The threat of a demon invasion loomed over the world, and corruption festered among humans. And the so-called summoned heroes?. Nah, it was just all part of a big lie.

Thus, Jack knew he had to grow stronger—and fast!

This is the tale of Jack Saunders, of his relentless journey, of his struggle against overwhelming odds to reunite with Anna.
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Additional tags: Slavery, ecchii, regression, cultivation, enhanced human, artificial intelligent, demons, seven deadly sins, artifacts, handsome protagonist, catgirls, foxgirls, time travel, angels, mythology, gore, grimdark...

I've read and watched so many isekai where the MC past life is just a loser. So, I thought, why not make the MC actually a strong, successful man? Why not make him a gigachad who forced his way into another world instead of getting transported there passively? Why not have him prepared himself with everything he could before he enters the other world?

Thus, this is my attempt in writing my own Isekai story where the MC's life on Earth actually matters, where there is an actual goal that is set even before the story begins.

Of course, it will still be full of anime/ isekai cliche that I love. Especially the beginning, it's inspired by The Rising of the Shield Hero. Therefore, some chapters can be dark and disturbing.
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Update daily

R-18 tag is for the dark and gore parts.