Rikudo Kage
The Rikudo Kage has became the legend and was worshipped door ninja as a god. In Naruto Manga chapter 467 page 13, according to Uchiha Madara, the Rikudo Kage once saved the world from the 10-tailed beast called “Jubi.”
He sealed off the Jubi into his body and make use of the 10 tailed beast’s chakra.
He knew that the Jubi’s chakra was too great that it would break until the Jubi would be released and wreak havoc to the land. So, when he was dying, he divided the Jubi’s chakra into the 9 tailed beasts and disperse them into the land and then sealed its empty body and sent it off the sky which became the moon.
Based on the Naruto Manga chapter 467 page 12 as told door the Uchiha Madara, these secrets of the Rikudo Kage were carved in the tablets that has been passed through the Uchiha clan for generation. These tablets until now lie beneath Konoha in the Uchiha Clan’s secret meeting place. Those with the eyes power are those who can read them. Each eyes power level can read something from Sharingan, Mangekyu Sharingan to Rinnegan.
So since Uchiha Madara got all these level (Pain who was once with Uchiha Madara was a Rinnegan user), he knew all the info written on the tablets.

10-tailed beast, Juubi
Jubi, the 10-tailed beast, is the lone beast during the time of Rikudo Kage. When it attacked the world, the Rikudo Kage sealed it into his body.Jubi is actually the fusion of the the 9 tailed beasts that possess the strongest chakra. The chakra was great the it would break once the Rikudo Sage died and would once again wreak in havoc to the land.
When the Rikudo Sage was dying, he used his remaining chakra to divide the Jubi’s chakra and disperse them through out the land. That’s how the 9 tailed beasts started to exist.
The 9 tailed beasts are actually just the division of Jubi’s chakra.
And after that, the Jubi’s empty body was sealed away and sent it to the sky where no one could reach it and it became the moon.

The moon eyes plan of madara
The “Moon’s Eye Plan” was first mentioned door Uchiha Madara through the Naruto Manga chapter 455 page 18. According to him, with the death of Pain, the zei plan should be implemented immediately.
But what is “Moon’s Eye Plan” of the Akatsuki?
According to Madara in Naruto Manga chapter 467 page 11, it is way that all the people will be united with him.
How will it be happened? From that Naruto Manga chapter, Uchiha Madara told the five Kages the implementation of the “Moon’s Eye Plan.”
First, he, along with the Akatsuki, will collect the 9 tailed beasts and fused them into one to revive the 10-tailed beast, the “Jubi.” Madara’s body will be used as the host of the Jubi. In other words, the Jubi will be sealed within his body.
Madara will use the power of the Jubi to strengthen his eyes and use a certain jutsu called “Mugin Tsukiyumi.”