Board Thread:Theories and Speculation/@comment-26358288-20150630121218/@comment-5407586-20150630140817

The only problem is that the Fifth Mizukage compared the genjutsu that controlled the Fourth to the one used by Danzō on Mifune: Kotoamatsukami. Unless you're telling me that a simple Genjutsu: Sharingan can reach the same level as Shisui's Mangekyō genjutsu, there's something fishy there. I would say that it was the power of the left Sharingan Obito hid under his mask until sacrificing it to use Izanagi against Konan. It sounds likely that it was the very same spare Sharingan Madara used in his old age after giving his Rinnegan to Nagato. If it was chosen by Madara as his spare eye, it's reasonable it was pretty strong.