Itachi (younger days): Inside the Nanga Temple's hall... underneath the seventh tatami mat from the right rear, you can find the secret hiding place of the clan. In this place, you can find the answer to the reason behind the existence of the clan's doujutsu....the true secret is recorded within. Once you have awaken your eyes, including myself, there will be three users of the Mangekyou Sharingan. For that to happen...kuku, it is a worthwhile reason to let you live.
Returning to present
Sasuke: Answer my question! Until you do so, the pain on your chest shall continue...
Itachi: ...did you miss my vital points on purpose...
Sasuke: Three persons...the other Sharingan user is...which person from the Uchiha clan?
Itachi: ...Why...are you concerned with such a matter?
Sasuke: After you, he's the next person I will kill...that's all.
Sasuke: You spoke of the existence of another person during the massacre of the clan....The Uchiha which you did not kill...that person is therefore the person assisting you. No matter how good you are, it is not possible for you to finish off the police squad by yourself.
Itachi: ...so you've noticed.
Sasuke: Who's that person?
Itachi: Uchiha Madara
Orochimaru's snake: The doujutsu power has a more sinister chakra than mine...same as Uchiha Madara of the past...
Returning to present
Itachi: One of the founders of Konohagakure...the first man who had awaken the Mangekyou Sharingan.
Sasuke: Founder? If it's that Madara you're referring to, he's supposed to be dead. Are you making fun of me?!
Itachi: ...Madara is still alive. To believe it or not, it's up to you.
Sasuke: Stop the bullshit!
Itachi: ...every human being relies on and is bounded by his knowledge and experience to live. This is what we call "reality". However, knowledge and experience are ambiguous, thus reality can become illusion. Is it not possible to think that, all human beings are living in their assumptions?
Sasuke: What are you trying to say exactly?
Itachi: Hmph... That "Madara is dead" is what you choose to assume. Just like how you chose to assume that I am a nice big brother to you in the past.
Sasuke reflects on the past when he played with Itachi.
Picture of the younger Itachi.
Itachi: The reason why I acted as the ideal brother to you... is to measure your "capacity"...
Returning to present
Sasuke: What happened that night... I could only think of it as an illusion when I was younger. That I was inside a terrible genjutsu... that's what I thought. But what was that?! It was clearly the reality!!
Following that, he Chidori from the back!
The face of the real (?) Itachi sitting at the back hit the side wall.
Sasuke: My eyes now are different from the past! I can see through genjutsu with my Sharingan!
Itachi: Hmph... you always use strong words, as usual. I take your words for it this time
The Itachi that was stabbed by Sasuke's katana disappeared.
Sasuke drew back the Chidori facing Itachi.
Itachi: ...however...Sasuke...you still do not possess the same eyes as me it seems
(Young) Itachi: And then you come forth to me bearing the same "eyes"
Sasuke: Hmph...then, you should use Mangekyou Sharingan and try to kill me! Or am I too much for you to measure your "capacity" now?
Itachi: Such great confidence...
Zetsu watches without letting slip of his presence.
Happy Zetsu: Both of them haven't been moving since the beginning.
Creepy Zetsu: That's because they are exchanging with genjutsu
Itachi: Mangekyou Sharingan...this eye is special...
Itachi: From the moment the eyes are awaken, it moves towards the darkness. The more you use it, it gets sealed
Sasuke: What do you mean?
Itachi: Mangekyou will lose the light eventually
Sasuke: Going blind...that is the price to pay for the power to control the Kyuubi right?
Itachi: Hmph...seems like you have listened to my words and went to read the stone board at the gathering place
Sasuke: Madara... what in the world is he?
Itachi: He was the first man to clutch the Kyuubi in his arms, my accomplice, my mentor and the indestructible man. And the only man to have exposed one more secret to the Mangekyou Sharingan. This person is Uchiha Madara.