First I would like to start off by saying nagato rinnagun was originally madara's. Here is how I came to this conclusion. Tobi claim to be madara until kabuto revived him. When nagato died bring everyone back to life from konoha tobi (pretending to be madara) says he was supposed to bring him back to life. When madara was resurrected by kabuto he says nagato had finally grown up.

Next I believe tobi killed madara. The only reason I believe this is madara knows of tobi exist and he didn't seem that thrill to know he was behind what was happening. I can't really prove it but i have a good feeling that's its a possibility.

Finally tobi is izuna uchiha. I know alot of u are thinking that izuna was shown dead in a coffin. Well i think that izuna was a fake. Izuna was suppose to be madara equal or at least very close to it. If that is the case I don't think izuna would willingly give up his eyes so his brother can keep his power. I think itachi was right n madara took his eyes by force. But i believe izuna took the eyes of another uchiha n went into hiding and disguise himself as tobi. If this is true that would explain why tobi has never been seen using a ms even tho he knows all about it n their abilities. He has to be an uchiha tho just for the simple fact he can using izangi. Itachi also says tobi(pretending to be madara) was a shell of his former self which would be true if he doesn't have his original eyes, but he would still be an elite ninja.