First, let’s talk about a fact that many people overlook: the great fire at the end of the Fourth Holy Grail War in Fuyuki was not because ’the Holy Grail was destroyed and thus exploded.’ Volume 4, Act 16 of Fate/Zero explains the mechanism very clearly—Kiritsugu Emiya ordered Saber to destroy the Holy Grail vessel that Irisviel had become. But what truly connected to the Greater Grail was the hole in the sky. The vessel was merely the key to open and stabilize the hole. When the key was destroyed, the hole did not close immediately; the black mud continued to gush out from within, causing the fire at the Fuyuki Civic Center and the surrounding blocks.
Kiritsugu paid for this misjudgment until his death.
And this mechanism of ‘vessel ≠ hole’ was taken to an even more terrifying variant by Zouken Matou in the Fifth Heaven’s Feel route. By applying the disaster logic of the Fourth War’s finale to the dual-vessel structure of the Heaven’s Feel route, one can deduce an ending more despairing than any route in the original work—and every step is based on confirmed settings, without needing to invent any new rules.
The division of labor between the two vessels: one holds the filth, the other opens the gate.#
Heaven’s Feel scene hf_15 already lays out Zouken’s plan. Ten years ago, fragments of the broken Holy Grail were implanted into Sakura Matou. After eleven years of worm modification, endurance training, and cultivation of compatibility with Avenger, she was made into an ‘incomplete Holy Grail.’ Her function is not to independently complete the Holy Grail ritual, but to serve as a vessel for the thing inside the Holy Grail—namely, Angra Mainyu (All the World’s Evils), which was summoned illegally during the Third War and then absorbed into the Grail—to attach to in the physical world.
This is why the black shadow that appears in the Heaven’s Feel route is not ‘Sakura turned into a monster.’ Kirei Kotomine explains the mechanism very clearly in the forest conversation in hf_15: the thing inside the Holy Grail uses Sakura, the incomplete Holy Grail, as its attachment, and can only act in the physical world as ’the shadow of Sakura Matou.’ The true body of the shadow is inside the Holy Grail; Sakura is its interface on the reality side. Killing Sakura does not mean eliminating the shadow—this constraint will become an unsolvable knot in the subsequent deduction.
But Zouken is not satisfied with just one interface. He also wants Illyasviel. The evidence is clear: Illya’s abduction is not simply a hostage situation; Zouken needs her to complete the ‘opening of the gate and incarnation.’ Sakura provides the attachment-type vessel, allowing the thing inside the Holy Grail to act in reality; Illya is the key component that advances the Holy Grail’s process from an ‘uncontrolled vessel’ to a ‘usable stage of realization.’ The functions of the two vessels do not overlap; both are indispensable.
Thus, the night before the finale of the Heaven’s Feel route is a countdown to a double rescue: Shirou Emiya and Kirei Kotomine must infiltrate the Einzbern Castle before Zouken completes the integration of Sakura and Illya’s functions; otherwise, Illya cannot be saved, and Sakura will completely become a part of the plan.
Deduction: If the ‘key’ breaks first#
Now, let’s overlay the disaster mechanism from the Fourth War.
The finale of the Fourth War tells us: when the vessel is destroyed, the hole does not close immediately. The vessel’s role is to open and stabilize the hole, not to be the hole itself. What happens when we apply this logic to the dual-vessel structure of the Heaven’s Feel route?
Assume a high-pressure situation: the war has progressed to the point where only the last two or three Servants remain, and Sakura, as the incomplete Holy Grail, has already absorbed the souls of multiple Servants. The shadow grows stronger with each Servant’s defeat—hf_11 already records the shadow’s ‘unscheduled feeding’; in hf_13, after her mental breakdown, Sakura develops a reversed killing intent: ‘If even Shirou is going to eliminate me, then I’ll take him first’; in hf_14, for the first time, she clearly wishes for Shinji to disappear, and the shadow immediately carries it out. Every step shows that the shadow’s autonomy is increasing, while Sakura’s ability to suppress it is weakening.
Then, at this critical juncture, Illya dies—whether because Zouken’s plan goes awry or she gets caught in the crossfire, the key needed for ‘opening the gate and incarnation’ is gone.
Now the situation becomes this: Sakura’s body has already been filled with enough Servant souls, the energy accumulation inside the Holy Grail is approaching the threshold, and Angra Mainyu is becoming increasingly active on the reality side through Sakura as the interface—but the key component to complete the ritual is permanently absent. The accumulated energy has no outlet.
During the Fourth War, the hole was in the sky; after the vessel was destroyed, the black mud gushed out for a while, and the hole eventually closed on its own. But the structure of the Heaven’s Feel route is different: here, the ‘hole’ is not opened in the sky, but in a living person. Sakura is the anchor point of that hole. Without Illya to complete the opening, this hole will not open normally, but it will not close normally either—it is stuck in an intermediate state: already connected to the Greater Grail, but unable to release its contents according to the proper procedure.
Finale: Not black mud, but ‘incubation’#
The next deduction requires a constraint from hf_15 that is easy to overlook: killing Sakura ≠ eliminating the shadow; the shadow’s true body is inside the Holy Grail. This means that even if someone tries to end everything by killing Sakura, the result will only be worse—once Sakura’s body is destroyed, the shadow loses its attachment interface on the reality side, but the energy pressure inside the Holy Grail is already too high to dissipate on its own. Without Sakura as this ‘controllable’ interface (though ‘controllable’ is a joke in itself), Angra Mainyu will seek the next outlet.
And the Fourth War already proved: when the contents of the Holy Grail cannot find a normal outlet, they will forcibly overflow in the form of black mud. In the Fourth War, the black mud spewed from the hole in the sky and burned down half of Fuyuki. This time, if Sakura’s body collapses under the high pressure, the outlet for the black mud will not be a hole in the sky, but a point on the ground—and then it will spread in all directions.
But an even more terrifying deduction goes in another direction: Sakura does not die, Illya is gone, the war continues, Servants are still being defeated, and souls are still being filled into Sakura’s body. The shadow grows stronger, and Sakura’s own consciousness grows weaker. The interlude ‘Misery’ in hf_13 already shows the path of Sakura’s psychological collapse under pressure—from ‘wanting to go see the flowers in the meadow with Shirou’ to ‘if even Shirou is going to eliminate me, then I’ll take him first.’ If this path continues, coupled with Angra Mainyu’s continuous erosion, Sakura will eventually transform from an ‘incomplete Holy Grail’ into a ‘physical body completely occupied by the thing inside the Holy Grail.’
This is not an overflow of black mud. This is incarnation—Angra Mainyu does not need Illya’s gate-opening ritual; it directly achieves complete manifestation in the physical world through Sakura’s body. What Zouken wanted was a controllable opening and incarnation, but without Illya as the stabilizer, what he gets is an ‘All the World’s Evils’ that no one can control, walking through Fuyuki based on Sakura’s flesh.
Why this ending is more despairing than the original#
In the Fate route, Shirou and Saber face a Holy Grail that can be destroyed—it is a device in the underground chapel, and destroying it ends everything. In the original Heaven’s Feel route, because Illya exists, Zouken’s plan can ultimately be stopped, and there is still a possibility of saving Sakura (albeit at a great cost).
But in the deduced scenario, both exits are blocked. Destroy the Holy Grail? The Grail’s interface is a living person, and killing the interface does not eliminate the contents. Complete the ritual? The key is gone. Restore Sakura? hf_15 already makes it clear that there are only two ways to restore Sakura: eliminate the shadow’s true body, or wait for the shadow to separate from Sakura and materialize, then excise it. But with energy continuously accumulating and Angra Mainyu constantly eroding, the shadow will not actively separate—it will only embed itself deeper and deeper.
The final situation is a deadlock: you cannot kill Sakura (if you do, the black mud overflows or Angra Mainyu finds the next host), you cannot save Sakura (without Illya to complete the ritual, the shadow cannot be separated), and you cannot leave it alone (with each additional Servant defeated, the shadow grows stronger). The endpoint of this deadlock is either Fuyuki being burned by the black mud once again—this time with no Kiritsugu to save a child from the ruins—or Angra Mainyu, wearing Sakura’s skin, remaining in the physical world forever in an incarnated form.
And the root of all this traces back to that illegal summoning by the Einzberns during the Third Holy Grail War. An anti-hero absorbed into the Holy Grail contaminated the entire system, and then, over the course of two generations, turned Fuyuki into its incubation ground. The Fourth War was its first large-scale overflow; the Fifth Heaven’s Feel route—in the absence of the key—will be its final hatching.
The mechanisms on which this deduction is based all come from the original text: the disaster mechanism of the finale in Volume 4, Act 16 of Fate/Zero (separation of vessel and hole), the causal chain of Sakura’s corruption and the dual-vessel mechanism in Heaven’s Feel hf_11–hf_15, and the cross-work Holy Grail contamination chain (Third → Fourth → Fifth). The deduction does not introduce any settings not supported by the original text.