It was a very well planned out theory that came out in middle of 06, it explained the relationship of Itachi, Madara, Sasuke, and the Mangekyou, this was before everything was revealed in spring 08 during Itachi/Sasuke fight. The theory has been disproven but at that time it was quite popular. It got so popular that people couldn't stop mentioning it during debates, it spawned countless mini-theories so the theory became hated and pissed off the mods. Basically the jist of it was that the uchihas had descended from a tengu god who slept with a byakugan wielding woman, and their lineage were the uchihas. And this father of all uchihas had been sealed in something, I forget what it was. And only 3 uchihas with Mangekyou could unseal him again, the 3 being Madara, Itachi, Sasuke.
Originally Posted by elitekage
It was a good try. And the theory was backed up with great mythological evidence. Click on the link amma gave, and read it if your bored. Fun fact, Itachi's Susanoo's full form is a yamabushi tengu.
A bridge towards peace; the will of fire still endures!
Rokudaime Hokage, Rokudaime Rikudou Sennin Uzumaki Naruto