A long time ago, in the history of the world, a single immensely powerful dragon lived. Befriended by twin brothers, kings even, the dragon helped create the Unova region into the peaceful place it is today.
However, not all was right. The two brothers, while twins, were not identical. Over the years, the two brothers began to value different things, and held differing philosophies on life; The younger brother sought ideals over all else, and the older valued the truth more than any other part.
This brought them to conflict, starting off with minor disagreements over how the region was run, and pulled them further and further into aggression with each other, arguing and eventually fighting. As they came to blows however, what has come to be known as the Unova Dragon found it could not side with either, nor with none.
So, in response to conflict between two beings it had pledged it's service to above all else, it split it's own power, and being, becoming two entirely different Pokemon. Reshiram, the Vast White dragon of Truth, a being of fire. And Zekrom, the Deep Black dragon of Ideals, a being of electricity. As the heroes fought, the two dragons began to fight also. However, as halves of a single being, they were evenly matched, and neither could triumph over the force of the other. As they battled, they laid ruin to the surrounding area.
Seeing this, the two brothers realized that both Truth and Ideals were merely two halves of a whole, and that you could have neither without the other. Upon this realization, they both declared that neither side was right, and began working to put Unova back into a peacful state.
This continued until the death of the two brothers. Rather than recombining into the original form of the dragon, the two dragons shifted alliegiance from the dead brothers to their eldest sons. These two sons, not having experienced so much as their fathers (nor seen the force of the twin dragons fighting), reignited the old conflict. A war broke out and Unova split in two, one side following the Truth and the other following their Ideals.
During this conflict, a new force made itself known; the being that would come to be known as Kyurem. Kyurem was the remnant of the Unova Dragon that could not split either way. Reduced to a withered husk, Kyurem remained at the site of the schism until a fallen comet impacted the location, creating a massive rift in the ground that would come to be known as the Giant Chasm. For unknown reasons, this revived Kyurem, gifting it with new life.
In contrast to Reshiram and Zekrom's fire and lightning, Kyurem was of Ice; an emptiness of the energy that once made it so revered. This emptiness drove it to the hunt, making a lair out of the Giant Chasm and raiding the nearby town of Lacunosa. In response to this, Lacunosa erected gargantuan walls, and made a policy of having nobody leave their fortified homes at night, a custom which continues to this day.
In an attempt to gain favour from the so-far neutral location of Lacunosa, the White King brought Reshiram to confront the "mysterious beast". In the battle that ensued, Kyurem demonstrated incredible power, forcing the White King into retreat, despite the relative weakness of the Boundary pokemon. A similar effort by the Black King was repelled in much the same way, if not even faster. Both Kings, on realizing that the other had also failed, put it about that it was interference from the other that had caused them to fail.
Somebody took note of this; An artisian, today unknown, realized that the emptiness and state of Kyurem gave it a measure of control over the other two order to capitalize on this realization, he began construction two artifacts. One, to control Kyurem, and direct it's frigid power at the warring armies. The other, a space warping artifact intending to fuse the dragons back into one. Grafting some of Kyurem's power onto it, it would allow Kyurem to hopefully force the dragons back into itself.
It failed.
While the invention was successful, being able to force Reshiram and Zekrom into a state of dormancy in the form of a small ovaloid stone the colour of their respective embodiments, they were now too different to be recombined. Kyurem could fuse with the dormant form of either of them, but it would only create a brutal mockery of what it had used to be and what it now was, forming limbs out of ice to make up for what it could not gain. Kyurem could also not fuse with both of them at once. It was also discovered during this point that the Dragons could be called out of their dormant formes by the human that they had bonded with.
However, the device to control Kyurem suceeded, to a point. It could direct it to differing locations, and even force it to attack targets, it would never quite hold the control over it that one of the Heroes held over their dragon.
This war dragged on for years, much of Unova being razed by the might of the two armies. In one clash, the island of the headquarters of one army was sunk into the sea. In retaliation, the massive fortress used by the other was destroyed utterly, and massive amounts of the surrounding area was transformed into a deserted wasteland. Many of the people fled to the north, leaving Lacunosa and Opelucid as two of the largest population centres in the country despite their nuetrality. Others fled to the west, across the river, taking refuge in the wilderness there.
The war finally ended many years later, with a climatic three-way battle atop the palace of the former regime, Dragonspiral Tower. Both Zekrom and Reshiram were laid dormant. Kyurem, who would have potentially coninued to rampage, had been wounded enough that it was forced to retreat, freed of the control of the device, back to its old hideout where it presumably remains to this day. Zekrom and Reshiram's dormant forms were placed in locations that were supposed to remain nigh-unreachable, in fear of what a third conflict could bring about. The two artifacts that held Kyurem's power were placed under the control of the Opelucid Dragon Clan and the Lacunosa authorities, where even now they are kept under heavy guard.
And the peace that fell across Unova from this lasted. The schismed country reformed. Abandoned settlements were recreated.
It all went well.
Until the day when a group called Team Plasma began to preach about Pokemon freedom.
A story we all know. And one for another day.