[Touko]

Originally Posted by ChainLegacy

It is the scene where Zouken asks Shirou to deal with the 'black shadow' towards the end of the route. Zouken talks with Shirou in the Matou household. He details the backstory and mentions that he and Tokiomi were using Sakura in hopes of gaining immortality. I suppose he could've made it up but it seems to have been a throwaway comment with nothing for him to gain from.

That's what Zouken said, but he was obviously lying. We know Tokiomi's goal is to reach the Root (rather than achieving immortality), from several source. He was just trolling Shirou.

This is the part you meant, right?

Spoiler:

To give a daughter to the Matou family means giving us a foundation to achieve our goal. Tohsaka is well aware of that. We both have the same goals. We will both turn ourselves into monsters to achieve immortality."

"Immortality…?
You and Tohsaka's father used Sakura for something ridiculous like that…!?"

"Of course. The Holy Grail War has always been a ritual dedicated to that end. That is the only reason we cooperated, and I am the only one who still lives.
To use the Matou successor to achieve our goal in the far future."

And also, Zouken was lying in saying that the Ritual was dedicated for achieving immortality, while he knew its ultimate purpose was for reaching the Root.