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The Old World
Before the Sundering, the universe existed as a single unified reality called the Old World. It was governed by 39 Sovereigns ancient rulers who each controlled a pair of opposing philosophical or scientific principles called Aspects. Light and Dark. Love and Hate. Life and Death. Order and Chaos. Memory and Oblivion. Strength and Weakness. These paired concepts maintained balance, and the Sovereigns kept them in check.
Everything in the Old World was connected by an invisible energy network called the Nexus. The Nexus was the fundamental thread linking all things. Aspects, Sovereigns, and the fabric of reality itself. It wasn't just a metaphysical concept; it was a tangible force that the Sovereigns could sense and manipulate.
What happened
The Sundering (also referred to as "the Honkai") was a catastrophe that tore the Nexus apart. The energy network that held the universe together shattered, severing the stable connections between paired Aspects. The world fragmented into 81 separate dimensional spaces, each one centered around a single, now-unpaired Aspect.

The shattered fragments of the Nexus crystallized into creatures of raw Aspect energy. The beings that would come to be known as Anima. Every Anima in the game is, in a sense, a piece of the old universe given form.
The World-Ender
The cause of the Sundering was the World-Ender an entity who used an "external power" to destroy the Old World. The 1st Sovereign, Victus, fought the World-Ender seven times and destroyed them twice, but the entity had achieved immortality through a "Pact of Rebirth" (also called the "Life-Death Contract") struck with the 5th Sovereign, Amstradath, who controlled Life and Death. This contract made the World-Ender impossible to permanently kill.
The game reveals early on that the player character IS the World-Ender. An amnesiac who was sealed in an illusory world for over 214 years. Before destroying the Old World, the World-Ender traveled with Parayaya (14th Sovereign) and Darsea (10th Sovereign) as friends. Something went catastrophically wrong.
Darsea's death
The World-Ender killed Darsea, the 10th Sovereign who controlled the paired Aspects of Memory and Oblivion. Because her authority encompassed Memory itself, her death had cascading consequences, the Aspect destabilized, and Oblivios disease began spreading across the Planes, erasing the memories of those it touched. The Sovereign of Memory dying and unleashing a plague of forgetting is the kind of cruel symmetry the game leans into.
Kumyo Kyo's response
Darsea was the younger sister of Kumyo Kyo, the 4th Sovereign who controlled Reality and Illusion. After her death, Kumyo Kyo sealed the World-Ender in the Plane of Illusion as a promise to his dying sister. Trapping them in a false reality for over 214 years. Kumyo Kyo then vanished. The brother who controlled Illusion imprisoned the killer in an illusion; the sister whose death unleashed forgetting upon the world.

Aftermath
The destruction of the Nexus had three major consequences:
Aspect separation: The paired Aspects split into individual concepts. Light was no longer tethered to Dark; Love existed without its counterpart Hate. Each fragment became a standalone principle
Plane formation: The 81 Planes formed around these separated Aspects, each becoming its own dimensional space with its own Novarch
Anima creation: The broken pieces of the Nexus became Anima creatures embodying specific Aspects. Collecting and understanding these creatures is how Animasters engage with what remains of the Old World's energy
What remains unknown
What was the "external power" the World-Ender used to destroy the Old World?
Why did they do it? The World-Ender's memories are gone, so even the protagonist doesn't know
What happened to the Sovereigns who didn't transition into Novarch roles?
Does Amstradath's uniquely unsplit domain (Life and Death still together) relate to the Pact of Rebirth?
Is the protagonist's amnesia a natural consequence of the 214-year imprisonment, or related to Oblivios disease?