Exerpt from ‘The Fall of Takhisis, vol. 1'

Era's ago the 7 gods created the world, each playing their part to maintain a natural balance. And for a long time all 7 lived in harmony. All gods were worshipped equally, except for one; Takhisis. For no living beings had a reason to worship the goddes of night and hatred.
Takhisis did not care, because she saw the true nature of these living beings and how things always ended; in war. Be it wealth, power or straight up ego, it always seemed to go in this direction, and she loved every second of it.
But still, over time, Takhisis grew resentful of her brothers and sisters and the lack of respect from the beings they had created together. This resentment grew and grew, festering inside, and one day sprung an idea:
"Why be neglected by these living creatures, while some unliving creatures do seem to appreciate all I have done for them? They are few and far apart because of the sheer number (and fear) of living beings. But they still take time to pray to me and even offer sacrifices in my name. Maybe it's time to even the balance."
And so Takhisis set on a path the other gods could not foresee. She wanted to bathe the world in eternal darkness and create more of the vile creatures and beings that seemed to favor her over the other gods. By the time the other gods figured out what was going on Takhisis had already gained massive power from the plane they all swore never to reach out to; the Abyss.
In doing so, the connection between Rehten and Otherworld was made stronger, and the 'Thin places' even thinner. Not only did Takhisis manage to create more monstrosities on Rehten, other beings with their own agenda's also slipped through.
The other Gods quickly decided their sister had to be stopped, and together they created Pandemonium and the Aeturnus Noctus Stone.
Pandemonium, a plane of madness, a great mass of rock riddled with tunnels carved by howling winds. It is cold, noisy, and dark, with no natural light. A plane with no hope of escape, where you could be driven mad by the incessant winds that sound like cries of torment.
The Stone; a key to this Plane that offers passage at any given time. Lest your intentions are to keep who-ever or what-ever contained in the Plane.
Together, the Gods marched on The Crescent Isle, the place Takhisis had claimed as her domain on Rehten, to plead for her submission. When she would not comply, a battle begun. Takhisis drew power from the Abyss and the creatures she had created aided her in battle. The battle lasted for days but eventually her brothers and sisters managed to imprison her in Pandemonium and the creatures she created scattered in the wind.
The gods knew Takhisis would never give up on finding a way back to Rehten, and did not want the few followers she had to be able to somehow use the Stone. They also did not want to cling to the key, worried sentiment would someday get the best of them and make them try to reason with Takhisis again.
Therefore they broke it into 6 individual shards, containing some power from each of them. Each shard was given to a "champion". The six champions, bearing a recognizable mark, were kings and peasants alike. Assigned to keep the shards safe and hidden from the world.
The combined effort from creating an entire plane and the battle exhausted the remaining gods in such a way they were forced to retreat to Mount Celestia. They would be able to visit Rehten again, but could nevermore call it their home. From afar they aided the beings that worshipped them, with their power being more evident when Otherworld is aligned with Rehten in such a way Celestia is nearest.
Over time, the story turned into a myth, the myth into a folktale and eventually into a fairytale you would tell your kids at bedtime.
But the shards remain, for they are without end. We can all only hope they do not fall into the wrong hands.