Rewind: Angra Mainyu's Will = The Shadow = Black Mud = All Evil = Grail Corruption... Where, Angra Mainyu (entity, not Will alone) = Zoroastrian God of Darkness.
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Chapter Begins:
Einzbern Castle,
Black read the contents of the letter in the Grand Hall, Medea standing by his side. Rider, with her head down, was standing against a wall not too far away.
The letter was from Kirei and mentioned his desire to test out Archer's potential through Lancer.
Of course, very conveniently, Kirei Kotomine's familiar hadn't delivered the letter until the fight had already broken out at the Homurahara Academy.
It was obvious to Black that Kirei hadn't wanted to share Lancer's movements until he was done with the little surveillance mission.
That played out against Kirei and Lancer, though. Because, in this way, they didn't learn of Rider's little plan of gathering life force through a modified Blood Fort Andromeda from the academy's students.
Also, given that Medusa was allowed to come and go as she pleased from the Einzbern Castle, she hadn't even informed Black and Medea about the reason for her sudden disappearance.
"Are you sure there were only two Servants there, Lancer and Archer?" Black asked Rider.
Medusa kept quiet, a dying Shirou's image reflecting in her mind repeatedly.
"You are needlessly worrying about him," Black said, smiling. "Rin is one hell of a mage. She wouldn't let Shirou just die, you know."
After a while, finding some sense in Black's words, Rider nodded. "Only Lancer and Archer…"
Black and Medea exchanged a meaningful glance.
"It doesn't make sense, Master," Medea said, conversing with Black mentally. "You gave the Assassin the best opportunity to ambush another Master and Servant. I guess Assassin's Master is no fool…"
Black's red eyes narrowed as he burned the letter with a flick.
Assassin getting summoned again was an event set in stone, he knew. Not because of the early death of Hassan of the Cursed Arm before the Holy Grail War had actually begun.
But because of Nothing Is Impossible.
When Black had stated that ALL EVIL MUST PERISH, rejecting a Rule from the House Common Room, he had combined it with Nothing Is Impossible.
By then, the Counter Force had already summoned a Ruler-Class Servant to deal with The Shadow before it ruined all humanity. However, the unthinkable happened, and Enkidu failed to destroy Dark Sakura before Ainz Ooal Gown took her away.
Because of that, and combined with Gilgamesh obliterating the Room itself, the Rejected Rule didn't come into the picture, and Nothing Is Impossible automatically took over the words.
Generally, given that no one agreed or denied the Improbable Impossibility of the event, the Skill shouldn't have worked, despite fulfilling all the other conditions.
However, as per the Lucky Intuitions and the Ad Infinitum, the Counter Force must have sensed the stirrings of the Fate on the battlefield.
Agreeing with Black, that the All Evil's end at that moment was not only Improbable, but also Impossible, it used the Skill to distort the Holy Grail War's rules, forcing another Agent to help the Ruler-Class Servant.
And that Agent couldn't be anyone other than an Assassin.
It wasn't what exactly Black had planned, but it was better than leaving things as they were, at least.
For Black, Dark Sakura must cease to exist.
He and Kai had no other choice but to help the Counter Force deal with the Angra Mainyu's Will now.
If everything had worked out as they had intended, then Rule Breaker had all but finished its job, finishing off Angra Mainyu's Will before it fully manifested as The Shadow.
Now that Dark Sakura had already come into existence, she would most certainly go after the Greater Grail, trying to incarnate Angra Mainyu.
If that happened, then instead of the Holy Grail manifesting in Black, as he was the Lesser Grail now, all the Mana would go to the Greater Grail for Angra Mainyu's incarnation.
As if this hadn't made matters worse, Ainz Ooal Gown was backing Dark Sakura as well.
Black let out a sigh within, rubbing his head. He had always known that his adventure to the Fate timeline would be filled with perils and unknown, uncontrolled, and unpredictable variables.
But that they would be to this degree, making even Ad Infinitum struggle, he couldn't have known beforehand.
Let's take one step at a time… like always… Black nodded within, remembering his roots. It doesn't matter who is Assassin and Assassin's Master. For now, the most important pawn is…
"Rider," Black said, smiling at her. "No need to hold onto it any longer. I know you have already gathered enough life force. Let's wake her up."
Medusa's eyes narrowed behind her eye masks.
Because of using a fully activated Blood Fort Andromeda on two Servants and a Mage, the process that should have taken days got completed in mere moments.
Also, because of Shirou's sudden appearance, Rider had to cancel the Noble Phantasm, or she could have squeezed even more Mana out of them.
Sighing, Rider followed Black and Medea to the bedroom, where Sakura was lying in pristine white clothes.
Rider put her hand on Sakura's chest.
With a thundering crackle, a massive spell circle appeared behind Medusa, red bolts of lightning snaking out of it, making Black and Medea step back.
A tiny spell circle appeared between Rider's palm and Sakura's chest as well.
All the red bolts of lightning, representing the condensed life force, entered Sakura through the small spell circle.
Sakura's body glowed red as if someone had pumped liters of fresh blood into her, her pale, bluish cheeks gaining a rosy texture.
Just as the two spell circles vanished, finishing infusing all the life force into Sakura, her body jerked. The red glow intensified on her, making everyone squint their eyes.
Then, everything quietened, and even the red glow died.
Just as they wondered if the life force supplied wasn't enough, tiny, black bubbles boiled around Sakura on the bed.
The bubbles enlarged and transformed, taking the shape of tiny figurines.
With a squeaking noise, under the shocked gazes of Black, Medea, and Rider, the tiny figures began dancing around Sakura as if worshiping her.
Slowly, Sakura's eyes stirred as she awakened —
— her pupils glowing like an infinite pool of utter blackness.