For as long as Itachi has been shown wielding these weapons I have been wondering how he could have found them and where he could have found them? Well after reading this latest manga chapter It has started to make me think about this possibility for him to have these weapons in his arsenal. My theory is that when he awoke his Mangekyou the weapons were created or given to him as well. Now before I go further into this please read this thread as a reference
Now in the thread it shows that both the Yata Mirror and Sword of Totsuka have no physical form. That being said It could be anything made from nothing. The thread above shows how the mirror has changed it size and we know that the sword can clearly change its size as well. Now if these objects can change their appearance then this is where I believe Itachi created them or received them. I hate using Harry Potter references but this one deals with the sorting hat and the sword of Griffindor. At first Harry pulls it out in the Chamber of Secrets when he needed the sword most. How convenient right? When the minister of magic informs Harry Potter that he cannot receive the sword of Griffindor because it is not Dumbledore’s to give. He goes on to tell him that any true Griffindor may summon the sword through the sorting hat. This is where I made the connection with Itachi. In the latest manga it shows Itachi at a young age learning from the old ruins that people were not concerned with any more.
In reality anything can be written on those slabs and he could have learned about the history of the weapons and what they can do. Even Hiruzen states that he had the knowledge of a Hokage at the age of 7. By the time he had obtained the Mangekyou his knowledge greatly increased and chance of him learning about the weapons greatly increases. The more interesting part of this theory is in the next manga page below
In this page Tobirama is talking about how the sharingan reflects the user’s feelings. Madara in this case has a lot of hatred inside of him and his Susanoo shows how much hate he has by giving him more power. Itachi’s Susanoo is very powerful but in a different way. His power is about destruction but about protection. Itachi has always been thinking about others before himself (mainly the village) but most importantly Sasuke. Everything he did was in order to protect Sasuke and when you really think about the Sword of Totsuka and the Yata Mirror they really aren’t weapons to be compared to Madaras Susanoo. Madaras main abilty is offense while Itachis is defense. With his personality I believe he unlocked the weapons a little after the Uchiha massacre. The sharingan reflects his feelings and seeing Sasuke must have helped unlocked them for him. The reason I also say after the massacre is because if he had them before Obito would have known and there would have been no reason for white and black zetsu to be surprised when Itachi wielded them. Of course I could be wrong; he could have unlocked them at the same time as Susanoo but never used Susanoo during the massacre. But that is an unknown factor. I know this is a long read guys but I do hope you enjoy the read. Thank you for your time.