You don't understand.
There can only be one heir. Everything goes to them. The second child...
Nothing and less than nothing. A cursed and hollow life, existing on the scraps of good-will and crumbs left over from the favored.
It would have been fine. Really, it would have been.
So why?
Father....
He thought this was best for you. He must have. You cannot imagine otherwise. The only reason is that he could not think that you would have been happy living forever in her shadow.
And so you were thrown into shadows that he could not, must not, have known were so much deeper still.
The Matou are long allies of the Tohsaka, two of the three Great Families. It only made sense to pass one child to the other, when the bloodline began to fade. Must have seemed like the most blessed of opportunities, a way for both to blossom, a rose and a rose by any other name, where before one would have withered.
He cannot have known anything about the Matou Magecraft. It's impossible. Easy to believe that he did not. Magi guard their secrets, held tightly to their breast, and only show the barest glimmer to any other, even those allied to them, or close.
The basement stinks of rot and filth and death, and it writhes.
"To practice the Matou Magecraft." Grandfather Zouken says, somberly. "You must first become of the Matou in truth, rather than just by name."
You don't want to.
There's so much you want to ask, to talk about, to delay the inevitable for even just a few brief moments more.
You don't have the chance. The next thing you know, you are engulfed, are flooded, consumed, submerged.
By the worms.
And as they prod and writhe and bite and chew at you, and you feel both inside and out as they infest your body and soul alike, you start to understand.
Grandfather, Zouken, he is and is not. The shell he wears. His body. He is the worms, and the worms are his flesh and his coursing and spilled blood, and his twisted soul, a legion of parasites feeding off of one another. The worm which walks as a man.
You scream, in terror and rage and fear, providing only one more venue for the worms to make their way within and perform their corrupting work.
---
[ ] You flee. You flee where you cannot flee. There is always an escape, of one way or another. You cannot run, and you cannot hide, but as long as you do not acknowledge what you see and feel it isn't real.
[ ] You fight. You struggle, wretched creature that you are, for your life. Your soul. The breath in your heart, and the blood pulsing through your veins. You don't want this. You hate this. You hate everything. Countless parasites, and you cannot pull them all free, but you must try.
[ ] You fall. Slipping down into the depths. It's the only way. The worms will devour your body, your mind, your very soul at his command. Unless you eat them first. Consume, drink deep, and accept without hesitation. It is the only way you can live.