Snowstar

Mark is a genius military weapon designer who was kidnapped by a mafia gang after refusing to work for them.
They forced him to design unique weapons for five years, until one day he successfully escaped only to get shot by his trusted comrades.
*Ding! You awakened a weapon system.
When he opened his eyes, he was in a different body and in a different world.
"You want to kill a wild boar without being detected? Forget about bow and arrow. Here, I present you this sniper rifle."
"You want to kill a horde of goblins from the sky? Forget about those flying carpets and brooms. Here, I present you the fighter jet." 
"You want to kill a dragon? Forget about taking the help of the S-class adventure team. Here, I present you a nuclear bomb."
Join Mark Spencer/ Lu Zhen as he brings the whole world to its knees with the help of his weapon system.
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Note: Author isn't born in an English-speaking nation and has no editor either. So, please be understanding if there are any grammatical errors.
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Author's current works: War Online, My Soul Card is a Reaper
Author's other notable works:  The Last Slytherin, The Sharingan Hyuga
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You can also support the Author through Patreon or buy coffee on Ko-fi.com

https://www.patreon.com/snowstar

https://ko-fi.com/snowstar5061

Weapon seller in the world of magic

Never in the world did Mark expect that he would feel like his trump card might turn against him.

What's more, there's indeed a selfish reason for that demigod realm mage to accept his deal. Until now, he only thought it was because the mage couldn't sense his cultivation rank.

Mark put his brain to use. He started thinking of everything he could to avert this crisis and change it into an opportunity.

Of course, he would never help them, not because they were demons and evil or something, but it is due to the fact that there is no quest prompt from the system. He has no gain from freeing the Abyss Lord.

After ten seconds of silence, he implemented the best idea he had in his head.

As both demons stared at him, Mark reacted by saying, "I'll help you with whatever you want, but I want something from you, Mr. Xalazar."

Absolute Authority