What is the true nature of the Holy Grail War? Is it magi proving their identity?
Is it the slaughter among Heroic Spirits?
Is it a nominally covert war, yet another day of 'peace'? Or the daily, household gas explosions?
No, none of these.
The essence of the Holy Grail War is, ultimately, a path connecting to the Root, the nature of glimpsing the Root, the desire of the greedy.
Originally, with Ryuya's power, he could destroy the leylines of Snowfield City, even those of Fuyuki City, stifling the Three Founding Families' desire for the Grail, and strangle both the Fourth and Fifth Wars in their cradle.
But!
Human desire is endless. Without Fuyuki City, without Snowfield City, there would be Natsuki City, Sunfield City.
Without the Three Founding Families, there would be the Four Creator Families, the Five Master Families. As long as the Holy Grail War ritual exists, as long as the path to the Root remains open, the Holy Grail War will never cease. It would simply restart under a different guise, as seen in the various variant wars across parallel worlds.
In that case, to seal the Holy Grail War, to isolate this unrealistic delusion, what Ryuya needed to do was simple.
Contrary to what he did in Camelot, where he opened the path to the Root, now he would close it.
"Beast of the Apocalypse, Devourer of Heaven and Earth."
A digestive system capable of consuming all things in the world was the skill that would utterly annihilate the Grail of All the World's Evils. After Ryuya devoured the Grail, surging magical energy circulated within him. But more than the magical surge, the path to the Root became clearer for him to see.
"This is the Root?"
Incomprehensible.
Unknowable.
Unobservable.
Residing in a dreamlike ocean, Ryuya felt as if he were adrift in a vast sea.
Alone, he completely lacked a sense of meaning, unable to even perceive his own existence.
But!
Beast of the End (SSS).
Legend says God created the world in seven days. The demon Satan was God's lifelong enemy. As the world-ending Beast of the End, the Beast of the Apocalypse becomes the sole ruler of an ownerless world.
Does the Root have an owner?
Does the Root have consciousness?
What is the Root?
Ryuya didn't know. He only knew he seemed to have grasped something now, an enigmatic existence similar to the Clow Card - The Time.
'The Root cannot be sealed?'
Within the world of the Root, Ryuya's gains were limited. Even with the Beast of the Apocalypse's skills fully active, he only comprehended three rules he currently possessed.
First, the Eternal Nation, allowing him to glimpse the path of life. Now he could not only see the lines of life but also the points of life.
Second, the Clow Card - The Time. He could control the flow of time forward or backward over larger or smaller ranges, not just reverse it as before.
Third, the regression of Midnight Lost Child.
The original Midnight Lost Child was created by a Crimson Lord, a device that forcibly reset the host's existence every day.
However, it accidentally touched upon the rule of 'Regression'.
Feeling he could use a similar skill, Ryuya originally could only apply Midnight Lost Child's regression to himself.
Otherwise, he would have wanted to give it, combined with Clow Card, to Souji, to at least delay her aging.
But with his understanding from the Root, he could actively trigger this rule on others using Midnight Lost Child. Although it couldn't cure Souji, it at least stabilized her condition.
After mastering these three abilities, Ryuya found he couldn't suppress the Root's closure.
Or rather, even if he could, it would only be a temporary seal, lasting about twenty years.
"Twenty years? That's enough. It happens to delays until the start of the Fifth War. By then, when I return, it will be different."
Taking a deep breath, Ryuya unleashed his full power, sealing the channel between the Root and the Earth.
Then, the notification for the scenario's end sounded in his ears.
[Congratulations, Host, for completing the variant scenario 'Land of Trials'. Evaluating... Score: SS (Epic).]
[Evaluation: As the Beast of the Apocalypse, you orchestrated the early commencement of the Holy Grail War, altering the fates of many who would have died. As a King from another world, you successfully protected numerous ordinary lives and saved the unfortunate fates of Irisviel, Matou Sakura, Matou Kariya, Tohsaka Tokiomi, and others.]
[Moreover, you devoured the All the World's Evils and closed the path to the Root. You have achieved nearly everything possible.]
[However, the still-young Beast of the Apocalypse ultimately cannot devour the Root. The most you could achieve is twenty years of peace. Driven by desire, future Holy Grail Wars will inevitably resume.]
[Congratulations, Host, for obtaining 'All the World's Evils (SS+)'.]
All the World's Evils (SS+): An abstract concept, originating from the world's sorrow. It possesses no inherent abilities itself, merely an absolute influence.
Can be combined with the Beast of the Apocalypse's 'Beast of the End', significantly enhancing the Beast of the Apocalypse's capabilities, and can corrupt Servants, turning them dark and controlling them. (Note: Cannot control Servants with absolute self-awareness or Grand Servant's Saint Graphs, e.g., Gilgamesh, King of Heroes, Solomon, King of Mages, etc.)
[Congratulations, Host, for obtaining the Noble Phantasm 'Evil Holy Grail (SS+)'.]
Evil Holy Grail (SS+): Unlike a wish-granting device, the Evil Holy Grail devours its owner's magical energy and twists the world's wishes. Normally, it consumes the owner's magical energy, storing it. The more magical energy it devours, the greater the explosive power it can release, even capable of opening a tunnel to the Root.
[Congratulations, Host, for completing 'Land of Trials'. Unlocked new scenario 'World of Spirits'.]
[Congratulations, Host, for obtaining points...]
As Ryuya awoke in the real world, he found himself back in his home.
And in his arms was his sister, Yukino.
"Onii-chan..."
Opening her eyes, her heart nearly leaped out when she saw Ryuya vanish in the Fourth Holy Grail War world, followed by the shattering of her own body.
Just like her sudden arrival in Snowfield City, Yukino remembered finally seeing her sister, Kaguya, and the others disappear similarly, along with Merlin and the others waving goodbye, saying 'see you next time' and 'do well in the competition'.
Back in reality, feeling Ryuya's warmth, Yukino, her heart finally at ease, concluded her journey to another world.
…
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A/N: After some thought, I'll temporarily set the rules based on mastery level.
For example, Mystic Eyes of Death Perception seeing lines is beginner, seeing points is intermediate, until the ultimate level, whether you see them or not doesn't matter, if you want to kill, it dies.
And reversing time by a day is beginner, controlling time forward/backward is intermediate, controlling it within a range is advanced, and time travel is the ultimate level.
Rules have no consciousness, like water and fire not interacting. But if they clash, it depends on the mastery of their users to determine strength, like a mountain fire meeting heavy rain… heavy rain extinguishes the fire, light rain fuels it.
Overall, the potency of a rule depends on the user's mastery level plus their own strength.