…not Rikudou Sennin but Madara himself. I didn’t anticipate that Kabuto’s trump card would be the last coffin that we haven’t heard anything about for 69 chapters (first revealed in chapter 490), but there’s always been a lingering suspicion that he might have resurrected Madara himself. The only thing that kept it from being the prevalent theory is the fact that Madara is alive — or so we’ve been led to believe.
From the look of things, the real reason why Kabuto was able to blackmail “Tobi” with the last coffin was because he could expose the fact that he’s not really Madara. Whether or not that’s truly the case I don’t know, but that is the cliffhanger this chapter left off on. I get the feeling that Madara’s younger brother Izuna is involved in some way — either as “Tobi” or this resurrected Uchiha that is presumably Madara — simply because I don’t see Kabuto breaking off ties with “Tobi” just yet. The two of them have a precarious alliance that will almost certainly end in betrayal at some point, but I find it a little hard to believe that “Tobi” would just let Kabuto pull something like this off without his knowledge.
I could be giving Madara too much benefit of the doubt, especially when I foresee Orochimaru becoming the ultimate antagonist again at some point, but it’s kind of hard not to when he’s proven to be as cunning as he is. Either way, I guess we’ll all find out soon enough who’s behind the mask.