50 Years ago, in various places around the world, Portals appeared. Initially, everyone was puzzled by the sudden apparition of Portals. 

A rainbow-colored hole that appeared on the air and delivered people into an unknown place? of course, each country in the world began to search a way to exploit those portals. Especially to solve population density, food problem, and recreation.

Though, there was one problem. Monsters wandered inside of those Portals, they attacked every living thing that entered portals. This was not limited to humans, but every kind of moving things that entered the Portal such as, dogs, cats, birds, and cars. Oh, right sometimes monsters got bored and wandered into Earth, the result? Just imagine hordes of cow-sized flame spewing lizard suddenly appeared in the middle of a street? Yeah... I bet people didn't want to eat steak after watching that incident.

Well, if there was a problem there must be a solution, right?

Yep, humans had a choice.

1. Defeat the monsters with firearms, grenades, or any other types of machinery and get nothing

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2. Defeat the monsters with traditional weapons and get a reward, let's just say... Gold coin? A drink that can heal any type of wounds? or Oils?

I guessed humans didn't like an easy mode, every country forbade the use of firearms inside of Portals, except for military personnel. Thus, the easiest steps to gain fortune was to have a combat 'Class', entering the depth of the dungeons, gaining levels, defeating the stronger monster, gaining more money and repeated it like 1000 times.

Well, yeah this sounded easy but the most important step was the first step "Have a combat Class."

From the data surveyed by the UN, as per 2018:

"1% of the world population is a hybrid.

10% of the world population is a spellcaster. 

25% of the world population is a fighter.

The rest of the world population is a specialist."

It wasn't like the specialized Class couldn't fight, they could, but they would not gain a single experience from killing monsters. Specialized Class gained a level by using their 'Class' Skill. For example:

Weaponsmith needed to craft 300 weapons using their Weapon Construction so they became level 2.

Farmer needed to produce 300kg worth of vegetables using their Cultivate Plant so they became level 2.

Tamer needed to capture 300 monsters using their Capture so they became level 2.

Breeder, yup that's me Asai Yu. I needed to breed 300 monsters using Mix so I became level 2.