One day suddenly, in various places of the world, dungeons appeared.
Initially, everyone was puzzle by the suddenly appeared dungeons.
Well, mostly youngsters, especially children, though.
Those entering the dungeons out of curiosity appeared.
That"s right, monsters.
In the dungeon was organisms never saw, it existed creatures that could only be called monsters.
These creatures attacked the living things that entered the dungeons.
This was not limited to humans, to every kind of living things.
To all the creatures that entered to the dungeons, dogs, cats, bird and the like.
Each country in the world began to search an explication to the dungeons.
Especially capturing and dissecting monsters.
However, what was found out was extremely slight.
Like normal creatures, monsters have organs such as brain and heart, there wasn"t big differences between both.
However, there was no DNA.
Thus, to investigate about the monster, they entered the depths of the dungeons.
When entering the depths of the dungeons, the monsters grew stronger than the outer layers.
Although the monsters in the outer layers were easily defeated, as approaching the depths it was harder, It has become impossible to go further one point.
From this, dungeons were considered as dangerous places.
Fortunately monsters never came out of the dungeon so it was forbid to enter, the world forgot the existence of dungeons.
But, that was a naive idea.
After a while, the monsters that never came from the dungeon, came out. Furthermore, countless of hordes.
Each country dispatched their armies, and did all they could, but they were pressed by the numbers.
Meanwhile, America was the first to use nuclear weapons.
Despite the collateral damage to the citizen, they successfully annihilate the monsters by the nuclear weapons.
Learning from it, China too uses nuclear weapons.
However, that brought a worse nightmare.
After a while, from the dungeons that were annihilated by nuclear weapons, the monsters overflowed again.
Moreover, there were more powerful monsters.
America tried to annihilate it again whit nuclear weapons, but it couldn"t destroy the monsters.
On the contrary, they became more actively and the damage increased.
It was the same in China.
After that, it was forbidden to use nuclear weapons to destroy the monsters.
But, it was too late.
America and China which used nuclear weapons in the flood of monsters, turned in areas where human can´t live.
Moreover, expanding the range of activities of the monsters and started attacking human beings.
Humans fought hard to overcome such monsters.
Originally they were using firearms to defeat the monsters, but due a certain event, and instead of firearms, they switched to a sword, spear, a bow and the like.
The event was that within the citizens attacked by monsters, instead of a gun they repel the monsters whit a metal bat.
The civilian was exhausted when he beat the first monster, but defeating the second and third, the time spend in defeating monsters decreased and the fatigue lessened.
Escaping from the raid of monsters to a safe place and receiving an examination at the hospital, that turned out.
Compared to those who just ran away from monsters, those who defeat monster heal their wounds faster.
That was something show more clearly when the person defeated more monsters.
To verify that, when defeating a monster, it was ordered to use a knife instead of firearm if there was room to spare.
The result was if you defeated it with firearm, nothing change, but those who defeat with knives or bare hands turned out to have improved their physical abilities.
Whit that result, humans" counterattack began.
Humans who were overrun.
Humans who learn the methods to fight slowly but definitely gain the power to defeat the monsters.