Let's see ... Here's my version of the opening:
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In a certain world where a certain event exist. Five centuries prior to the story, the gateway between different worlds were opened. The main world government tried to make it a secret and said to the people that the mysterious phenomena are in investigation. 

The actual cause of the gateway itself because of the project of our Government to find a new world for human to live. They realized the population kept growing and the earth unable to contain them anymore. Pollution that made by human-kind already damaged their world and the nature finally punished them. 

At another sides, several worlds get roughly the same thing as their world started dying and they happened to do the same project.

However, the effect was unexpected. The project they made happened to collide each other and created the mysterious gateway, which however can't be access for some reasons. It kept like that for some day before it starting to get big and engulfed everything. For a moment, the worlds 'destroyed'.

The moment the world come back to light, everyone surprised. The land they stood changed, Everything changed, yet there's a familiar feeling when they saw this new scene. The peace only go for a while before it went to chaos when these different races meet each other. Misunderstanding happened and first war happened in this new world. Due differences between the races, some races started dominating the war.

That before Ancient Monsters called 'Weapon' appeared out of no where. According to theories developed by intellectual races, these 'massive bio-mechanical engines of destruction' was created by the Planet to stop the war itself. Unfortunately, the races in New World was considered as 'target to eliminate' by 'Weapon'. The war that happened before stopped and the genocide began 50% of the population was eliminated in process just by 7 of them.

To counter it, every races set aside their differences and worked together to handle these Weapon. Their combined powers managed to push 6 Weapon and made them gone to somewhere. The Weapons crystallized themselves awaiting the time the Planet need them once more. but no one know the reality.

However, The Seventh Weapon went haywire and decided to destroy these combined power by going right at the Headquarter of this alliances on Caelifer. The races decided to take it head-on, but it isn't easy and the price just too big. Lots od them dead and Caelifer's geography changed a lot due the battle.

With the destruction of the final Weapon, the races celebrated the win by decided to make the CEBSB (Cultural Exchange Between Species Bill) and named the new world as "Uno Mundus". The world finally achieved its first peace.

Now year 2550, 500 years after Uno Mundus created, but there's still lots of mysteries hidden by the world. Lots of places to explore. Here the story begin. What will you do?

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Does it work?