An explanation of how the fate system works in Teyvat and how the Heavenly Principles control everything that happens and everything that will happen.
An important clarification before I begin: This post is a personal effort that took me two months of research and information gathering. The post is based on my personal understanding of the texts and my own connections between them. Some points might just be my own speculation/theory, given that the information I found isn't abundantly clear on them.
Let's begin the explanation.
A brief primer on the civilization of Remuria, ruled by King Remus: This civilization's entire history and research were built on attempting to forge their own destiny, outside the control of the Heavenly Principles.
(A quick note about the people of Remuria: the word "Fortuna" in their texts means "Fate," which is the system they believed in.)
The people of Remuria researched the inner workings of fate in Teyvat in great detail, and half of this explanation will be based on their findings.
In the beginning, over 7,000 years ago, after the Heavenly Principles took control of Teyvat, they created the False Sky. They wrote the future and destiny of both mortals and gods in the stars of this False Sky. Meaning, every outcome, battle, or catastrophe that happens was written thousands of years in advance by the Heavenly Principles.
These stars, where the Heavenly Principles wrote the future of all creatures, are what we currently know as Constellations. Mortals, gods, and angels alike all have their own Constellation, crafted by the Heavenly Principles thousands of years before their birth, and through which they manipulate their fate.
“After all, 'fate' is merely the manner in which the present ruler of this world plays with living beings.”
— Source: Neuvillette's Character Story
Conceptually, fate works simply: the outcomes written by the Heavenly Principles in the stars of the False Sky will happen in reality. No matter how hard you try to alter the destiny written by the Heavenly Principles, you will not succeed. The best example is King Remus. After foreseeing his future and realizing that his civilization, Remuria, would fall, he tried to change fate through a meticulous plan. But ultimately, his plan to fight fate was a part of fate itself, and Remuria fell because of it.
"Yet fate's decree was not for even such as gods to defy, and to even attempt to escape destined judgment was a mortal sin... Such is fate's cunning that it uses the very powers that rebel against it to its own ends. When the day of destiny arrived, Remuria, the grand immortal city, faced an inevitable end."
— Source: The History of the Decline and Fall of Remuria - Vol. II
Meaning, the outcomes written by the Heavenly Principles will happen no matter what. These outcomes cannot be changed unless something powerful from outside the system is introduced (I'll touch on how to change fate in a bit).
Returning to the people of Remuria, who read these fates and categorized them into three types:
1. Fate of the World
This type of fate determines the rise and fall of civilizations. It is the reason behind the fall of Hyperborea, Natlantia, Remuria, and Khaenri'ah. Most major civilizations fell due to the "Fate of the World" system. This type is the most powerful and the hardest to alter. Every move and step a civilization takes falls under this system, making their doom inevitable and impossible to change. Even the civilization of Khaenri'ah could not escape this system and was annihilated exactly as the Heavenly Principles had planned.
"Kingdoms rise and fall, and when a civilization is annihilated, a new one will be born after from the ashes, which these books refer to as Fortuna."
— Source: Enigmatic Page (I)
(And yes, Khaenri'ah was not destroyed solely due to King Irmin's actions, but also because the Heavenly Principles had written a destiny of destruction for Khaenri'ah from its very beginning).
Most things that occur in Teyvat are monitored and planned by the Heavenly Principles through the fate system they created. This brings us to the second category:
2. Fate of Mortals and Gods
The will of every creature is subordinate to the destiny written for them by the Heavenly Principles. This means every action taken by a human or an Archon/god does not stem from their own will or desire, but from what the Heavenly Principles have written for them. This applies to both mortals and gods; both are bound to the same system.
"Therefore, they cannot be regarded as humans equal to us. Their choices do not come from their own wills, but rather the course charted for them by Fortuna... They have not the freedom to make decisions, and all they will do is play until the final note that was long ago written for them..."
— Source: Harmost's Notes (II)
For this reason, all actions of living beings are subservient to the will of the Heavenly Principles, and you cannot deviate from their will no matter what you do. Furthermore, the harder you try to fight or change fate, the more entrenched and inevitable that fate becomes.
"... The louder their music, the more resolute their wishes, the firmer the order of Fortuna becomes... None can escape..."
— Source: Harmost's Notes (II)
To prevent any human with strong willpower from turning into a Descender, the Heavenly Principles created Visions. Literally, if you had a 5% chance of changing your fate, a Vision drops that percentage to 0%. Meaning, once a human receives a Vision, they become completely subjected to the fate crafted by the Heavenly Principles. They can neither change their fate nor become a Descender in any way, because their will now belongs entirely to the Heavenly Principles.
"To receive a Vision is to sell oneself to the "fate" of this world - to Heimarmene, and to evermore lose the chance to walk the correct path."
— Source: René's Notes (Narzissenkreuz Ordo)
Visions and Fortuna are considered interconnected. Fortuna is fate, and Visions are what anchor the will of creatures within this fate more securely, binding it to their souls. If they lose their Vision, they lose their ambition/will, because upon receiving the Vision, their will became tied to it. There is no longer any chance for their will to transform them into Descenders who could threaten the Heavenly Principles' plan.
In short: All creatures follow the destiny written by the Heavenly Principles. However, some humans who possess stronger-than-average wills and have a chance of becoming Descenders are sent Visions to control their will and prevent them from reaching the level of a Descender. Visions also serve other purposes and tasks beyond cementing fate, but these other aspects related to Visions remain unknown.
How the Fate System Works Internally
Now we'll talk about the most complex part: how the fate system works from the inside.
In reality, the internal workings of the fate system are much like an advanced computer system, and the processor of this computer is the Irminsul tree. Irminsul uses available information from the past to write the future, according to the algorithms and rules inputted by the Heavenly Principles. It writes the destiny and future of mortals and gods based on these set rules and data.
When the Heavenly Principles took control of Teyvat over 7,000 years ago, they created the Ley Lines to maintain and protect the planet's new ecosystem. They connected the Ley Lines to the False Sky in a way that allows the Irminsul tree to receive information from the roots of the Ley Lines, which are connected to the stars of the False Sky. The connection between them isn't physical, but digital—something closer to Bluetooth. The Ley Lines send information about the future to the stars of the False Sky, which in turn hold an unchangeable code.
“The Ley Lines were created by the Heavenly Principles, and it is only with their protection that Teyvat is able to nurture all forms of life.”
— Source: Mavuika (Archon Quest Chapter V)
"I once thought that through him, I could discover how the one seated upon heaven's throne converted the Ley Lines into a girdle that defends the planet. I believed that once I understood the secrets of its operation..."
— Source: Kukulkan ("An Omen of Annihilation and the Final Entreaty")
"Do you ever get the feeling that the Ley Lines have a regular flow, similar to the way that celestial bodies follow fixed orbits? If we were looking down from on high, I wonder whether we'd find that the Ley Lines are just the reflections of the stars upon the earth?"
— Source: Layla ("Competition on the Sands")
To put it simply and clearly:
Every Constellation in the False Sky has a code. This code strictly binds it to a specific destiny and nothing else. Therefore, the Irminsul tree creates a future based on past information that fulfills the conditions of the Constellation's code.
For example, if someone's Constellation is always linked to a destiny of dying by poison, all Irminsul does is write the sequence of events that will lead to this person dying by poison. Meaning, Irminsul does not change the outcomes written in the Constellation; it merely writes a destiny that will lead to the outcomes written in the Constellation's stars.
Of course, this is just an example, and the concept isn't only about death. Every Constellation has a specific, defining feature that must always be present in its owner's future. Take the Constellation of the Knight of Boreas, for instance, whose destiny must always lead to the position of the Grand Master of the Knights of Favonius.
Irminsul and Reality
Returning to the main topic regarding Irminsul's relationship with fate: Irminsul doesn't just write a large part of fate; it also turns that fate into a reality and a truth that the person experiences. Irminsul is responsible for weaving the destiny written in the stars into reality. Meaning, whatever Irminsul fabricates in reality becomes truth. For example: if someone died in the past, and Irminsul wrote that this person would return to life, that person would actually come back to life.
(Side note: When people die in Teyvat, their souls go to Irminsul's Ley Lines. Furthermore, the planet's ecosystem relies on Irminsul, and the memories of mortals and gods depend on the information held by Irminsul).
Summary of the System:
The Heavenly Principles write the outcomes and conclusions for every creature's destiny in their stars/Constellation. They set algorithms and laws by which Irminsul operates, so that it can write the future that will lead to those outcomes written by the Heavenly Principles in people's stars/Constellations.
That is why when Rukkhadevata met Nabu Malikata for the first time, she introduced herself like this:
"I am the owner of yesterday and the master of tomorrow morning"
(Meaning, the mistress of the past and the future.)
— Source: Scroll of Streaming Song
The goal behind all of this for the Heavenly Principles is to maintain absolute control over Teyvat's future and avoid any disasters that might lead to the system's collapse. The Heavenly Principles don't just control the memories of all creatures in Teyvat, but also their wills and their entire futures.
Who Can Defy Fate?
Now we'll talk about who doesn't submit to the laws of the fate system, why they don't submit, and how they can change fate.
Outlanders: Beings from outside Teyvat are not bound by Teyvat's fate system. The Heavenly Principles wrote the destiny of all Teyvat's creatures, but they didn't write the destiny of creatures from outside the planet. Meaning, if someone from outside the world enters Teyvat, they aren't bound by the laws of fate because they weren't born inside Teyvat. They are considered beings from outside the system, and the fate system doesn't recognize them.
The Abyss: The power of the Abyss and Abyssal monsters are not subject to the laws of fate, though they are indirectly affected by it. Even creatures that merge with the Abyss, like mortals or gods, become free from Teyvat's fate system and gain real free will. But unfortunately, this will is influenced and corrupted by the Abyss.
Dragon Sovereigns: The Dragon Sovereigns are not subject to the fate system, and the stars determining the Sovereigns' destinies are located outside the False Sky, out in the real universe.
Descenders: Descenders are the ones completely unaffected by Irminsul's control over Teyvat's fate, and they don't have any Constellation in the False Sky. (Just to clarify, a Descender doesn't necessarily have to be from outside Teyvat. Even if they were born in Teyvat and never left it, they could still become a Descender, because the requirement to become a Descender is possessing the strong will to change Teyvat's fate). The reason they can change Teyvat's fate is that their actions affect fate, but fate does not affect them.
Meaning, if a Descender kills someone who was supposed to play an important role in future events written by the Heavenly Principles, that event will drastically change. Here, the Descender would have impacted a future event and altered part of the destiny written by the Heavenly Principles.
This example applies almost exactly to the others I mentioned, like the Abyss and the Dragon Sovereigns, but it differs slightly because they must possess sufficient willpower to actually change part of fate. Because even though they aren't affected by the fate system, they generally can't affect it either—meaning neither side can naturally influence the other. That is why willpower plays a massive role in the matter of changing fate.
How the Heavenly Principles Steered Khaenri'ah to Destruction
Over 500 years ago, the people of Khaenri'ah were summoning creatures from outside the world of Teyvat to utilize them in any way possible:
"In anticipation of the arrival at their Kingdom of gods from beyond the so-called ocean — or rather, the arrival of beings who could transcend the gods"
And while they were summoning creatures from outside the universe into Teyvat, the Heavenly Principles had allowed the Twins to enter through the False Sky, and permitted the people to summon Lumine:
"Plus, she only came to this world because the heavens responded to the summoning"
— Source: Scaramouche ("The Night-Bird Falls at the Curtain’s Call")
But the people of Khaenri'ah didn't know that the reason Lumine reached them was because the Heavenly Principles allowed them to summon her! The Heavenly Principles also inputted Lumine's data into the Irminsul tree, making Lumine a part of Teyvat and intertwining her destiny with the world's fate. All the Heavenly Principles did next was wait for the catastrophe, while Irminsul completed the task and steered Khaenri'ah's fate toward destruction.
And, of course, we know what happened next from the story of Rerir and the quest "Space and Time for You." This is the available information on how the Heavenly Principles steered Khaenri'ah's fate toward ruin.
This concludes the explanation! If you have any questions, leave them in the comments, and I'll try to answer all of them.
I originally posted this three months ago on Twitter in my native language. This is the English version of that post. I used an auto-translator to convert the original text to English, and then reviewed it to correct any errors that might result from auto-translation. Because of that, I think everything is accurate (but if there are any mistakes, please let me know). If you have any notes or points I forgot to mention, I hope you share them; I would be thrilled to read them!