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

The mage's smile widened upon realizing that the woman was actually willing to sacrifice her life. He didn't waste any time in striking the deal, "I'm willing to go through the Blood oath to the elemental spirits."

"Not a Blood oath, but do a soul oath, Vexiron," replied the Elven High Priestess, not intending to trust him.

There are three types of oaths one can do to the heavens.

One is the oath of cultivation with the heavens as the witness. Breaking such an oath will incur a lightning strike from the heavens, but it can be avoided when one enters the demigod realm.

Second is the oath of blood to the elemental spirits. Breaking this oath will incur the wrath from the elements of nature such as fire, water, lightning, earth, and wind. The forces of nature keep on trying to kill the oath breaker, but in this process, it can destroy the world too.

System's intervention