Rin, Shiro, and Saber gathered in Emiya's Residence.
They had just saved Shiro from Illya and lost Archer during their escape from Einzbern's Residence.
"What do we do about the mystery servant?"
Rin spoke first, she was depressed about the series of events. She lost her servant meaning she lost the war. Now, she remained to help Shiro and Saber.
"He's either an assassin or a caster. Either way, those type of servants needs preparation to fight. So, we can only wait for them to come at us"
Saber summarised she was sure she can win a head-on battle.
However, a good strategy can change the outcome of a war. So, she's being careful, she can tell this mystery servant is powerful.
***
In front of the Matou household, Cael in his disguise entered the house using his Halo of resurrection to conceal his presence from reality itself.
He directly went to the basement of Matou's and saw naked Sakura drowned in the sea of worms.
'Such sight reinforces my will to reach the root and become untouchable from humans and will of humans '
As a magus, he can tell how desperate Zouken is, he needed a vessel but then, he can't just accept anyone.
Sakura is just talented and unlucky which led her fate to become like this.
Still, he didn't mind such methods.
After all, humans in the end are animals too.
Cael is no different from Zouken.
Zouken defiled Sakura's body for his own benefit.
Cael defiled Soren's soul for his own benefit.
Cael acknowledged this fact after seeing Sakura in such a state.
At that moment, something snapped in him. He decided to check this later.
Mist surrounded the room.
Sakura saw a man covered in mist, she can't identify the features but she can tell someone are there.
"Sakura, I'm master of Assassin. Rider made a contract to save you. Do you have anywhere else to go?"
Sakura heard a deep voice, she shook her blank eyes.
"Very well, how about this? I will take you as an apprentice and cure you, I know you don't trust anyone including your family, I don't trust another soul so, I kind of understand that. But this will be your only chance to escape from your Fated tragedy. "
" I will save you from here regardless but that doesn't completely help you, your body is a mess and is altered by Zouken, it's only a matter of time before you become a mindless monster if Lucky you will become a psychotic monster by your own will"
Sakura saw the misty appearance of the magus and the worms crawling on her body.
The Magus's words state that her fate is sealed either way even if she escapes now.
A deep sense of resentment rose in her, never had anyone come to save her, the only person who loved her died. She's hopeless, she has been r*ped by her brother constantly if not him, it's this worms.
Her Rage, resentment, and the future where she will lose herself made her angry and hateful.
"....Humans are animals, selfish animals. It's only natural to feel hateful when you have been left alone but that doesn't mean you have to give up your life for that resentment.
Live for yourself. Your life is more important to you than anything in this world.
Your family and this world are not worth anything compared to your life.
Be selfish, your life is the only thing that matters, everything else is worthless in this world"
This is his personal motto. For him, it has always been about himself.
Sakura asked in an emotionless voice "Then, why do you care about me? You are doing more than what you are supposed to "
"I admit, I'm no hero or a devil. in my eyes, Good and evil are just tools of control and manipulation.
I still stand by what I said, now, isn't that easy to figure out why I'm giving you this offer? You are a magus with special potential, I could study your potential when you are with me close by. Though, I don't handle things like Zoken or most of the magus"
Sakura lay there in the pit of worms crawling all over her body.
"Heh~, then I'm just switching my torturers. You are no different from my Grandfather who tortures me and my father who sold me. You are right, humans are selfish animals"
There is a hint of mockery in her tone but Cael didn't mind it.
"You can judge me after a year, even if you still feel the same, you can leave the apprenticeship, I will protect you from other magus even if you left. So, what do you say? Would you like to be my apprentice?"
Sakura stood up then said, "Take me away then, I am willing to be your apprentice but you have to promise me that you will kill Grandfather right now, right here."
"That's easy, now then, my tragic apprentice. Time for your Lesson number 1, Your master is the best magus in the world, remember that"
Sakura saw the clothes appear out of thin air and cover her. Then, she flew till she reached the ceiling just when she thought she would be hitting the ceiling. She phased through it.
Finally, she's above the sky looking down on Matou's Residence.
She saw the whole residence collapse in a second crushing everything into oblivion.
*Boom*
A swarm of insects flew out clustering into a wraith. Zouken Matou formed in the air when he looked up at the air. He saw Sakura flying along with some unknown entity.
He laughed madly and said "Kukuku~, interesting, you have gained good support sakura but that doesn't mean I still can't control you. Now, come down obediently"
Fear struck Sakura out of instinct and Zouken is also saying the truth. She knows she can't defy him.
However, the foggy hand stopped her from trembling.
"Know Your Place, Third-rate"
Cael drew a rune on the sky with purple prana from his fingertips.

As a magus who dealt with curses, Zouken can feel the potent curse from the rune.
Fear spread across his entire being, his soul will be destroyed like a glass the moment the curse activates, he can feel it.
His body turned into countless insects and scattered as fast as they can.
The rune stayed there however, Zouken felt his soul shatter, his already fragile soul couldn't handle the powerful curse rune and he died.
'I should name this rune 'Posion Demise' for namesake. With a potent curse like this, that targets the soul, I can kill most people with this curse. No matter what, I will remember your marvelous contributions, Soren'
Sakura felt relieved when she saw Zouken die in front of her eyes.
'Senpai should be safe now that grandfather is dead. I don't have to worry about him'
"Alright, let's get you to safety"
"....Sensei, the Matou family has incredible knowledge stored in the basement"
"Zouken's Magecraft is inferior to mine, it's useless"
Cael's answer didn't surprise Sakura. Her new teacher can fly, crush things, create things, and finally use a strong rune all without the use of incantations.
Simply, he's above first-rate magus.
She just wanted to show sincerity.
They both went to a new residence outside of Fuyuki City.
After going there, Cael transferred Rider's command seals to Sakura.
Rider manifested and thanked Cael.
"Thank you very much, I would have appreciated it if you took me with you. That doesn't change that you have stood by your promise"
"The contract with you is complete. Now, Sakura, lie down on the medical bed"
Rider got defensive "Why?"
"I took her as my apprentice. Even though I saved her from Zouken, Her body is altered heavily. Crest worms are still crawling through her body now as we speak. At this rate, she will lose herself and Zouken's personality will take over her body. Also, I found parts of the cursed grail, so she's a vessel for the holy grail"
Assassin appeared and asked, "What do you mean by cursed grail?"
"I don't know, I have to heal her and extract the cursed grail parts from her. Then, I can tell anything. For now, the grail in her body is cursed "
Rider didn't care about the grail but after realizing how horrible her master's state is.
She let Cael analyze and heal her.
Cael did a proper analysis and then, he began to remove the crest worms, Zouken's influence, and shards of the Holy Grail.
After healing her, he asked, "How do you feel now?"
"I feel better than ever I could be, Thank you Sensei, I take my word back, you are not like my grandfather and Father"
"You are that miserable huh? Your treatment is not over, I removed the maximum foreign influence on your body. The amount of pain you are experiencing now after treating you is like a stab wound.
Your bloodline is mixed with Matou's bloodline a lot. I have yet to remove that influence
You still need a few months for you to be perfectly healthy. Until then, don't practice any magecraft"
Cael is serious when he said he's willing to take her as an apprentice. Sure, he primarily wants to experiment with her origin of imaginary numbers but that doesn't mean he will neglect her, he is willing to treat her as an apprentice and teach her.
"Now, Rider, Assassin, and Sakura. I have something important to tell you regarding the Holy Grail War"