The dual planets were intertwined in each other's tragic destiny, dancing an eternal rondo around a pitch-black great sun. 
In the beginning, one of the planets gave rise to humans. These people looked down, and named the land below "Izumo." These people looked up, and named the world above "Takamagahara." 
One day uninscribed in the annals, the Yaoyorozu-no-kami descended from Takamagahara. These demonic beasts named Kami overturned the skies, burned the rivers and oceans, and shattered the land — People realized in horror that it was not an invasion for rulership, dominance, or plunder. The evil Kami came only to kill. 
As the planet stood on the brink of death, Izumo embarked on a Kami-slaying journey, exhausting the might of the entire country to slay the calamity "Sovereign of Revelation" and forging the very first Edict Edge using its bestial body. 
Giving the Kami a taste of their own medicine, the wielder of these swords can recite the mantra contained within the blade to master the divine power of Takamagahara, and use it to combat the evil Kami and save the masses. 
From then on, Izumo started its long and arduous crusade, exchanging inordinate lives lost for the fall of one Kami after another, shattering countless mortal blades to forge the twelve Sentinels. 
In the merciless fight for survival, Izumo used the might of the Edict Edges to light up their uncivilized world of darkness in less than ten Amber Eras, building cities of dazzling neon lights. 
That distant Takama Divine Realm, once so far away, became so close within their reach —
Yet history suddenly stopped there. The existence of the two planets instantly evaporated, disappearing into nothingness. 
Nowadays, the past of the border planet Izumo can only be gleaned from the scattered whispers in the universe. Academics hold various views regarding its disappearance, but none can solve the mystery. 
Izumo's history should have been a long flowing river, but it was severed in one cut, and all its past and future voided on emptiness' other shore. 
Did it never exist, and was it no more than a fabricated story? Did its histories never occur, and it ran aground on the beach of reverse causation? Was it yet another appalling experiment by Dr. Primitive, or was it a sign of the Voracity returning from the end of the Cosmos? 
Only that pitch-black great sun knows the answer, yet THEY remain silent, never speaking. 
Because everything that had happened will one day regress to the end, and everything that had ended is guaranteed to happen again. The universe undergoes an eternal recurrence under THEIR shadow, and Izumo is nothing more than the footnote for an ellipsis.
 
(Source: Honkai Star Rail Youtube Channel)

Honkai: Star Rail - Rondo Across Countless Kalpas