Part 1: The Ten Tails, Son of Kyuubi
Sixty years before the Nine-Tailed Fox attacked Konoha, the Tailed Beast roamed and attacked wherever human malice grew and festered. With each person it killed it's power grew even larger. It entered the Land of Fire only to be confronted by the Uchiha Clan. The fox lived among the Uchiha for a few months killing their enemies and even some of their own. Eventually it's power surpassed even it's own control. It released this chakra into the form of a second Nine-Tailed Demon Fox. The original Kyuubi left soon after. His son was then confronted by the Uchiha, most notably Madara Uchiha and his brother. The entire clan used a Kinjutsu to try and subdue the new fox. The kinjutsu gave the fox more and more power, even more than it's "parent" and so a tenth tail formed. The foxes' fur turned ink black and it's once malicious attitude was replaced by a very smart and very caring attitude, and so the 10 Tailed Fox was born.
Part 2: 15-years ago; Amegakure
The Land of Rain had been thrust into civil war. Yomi Uchiha was a native of Amegakure and one of it's most prestigious Jonin. Pain had just won Amegakure and was finishing off the pockets of resistance to his authority. Yomi walked into a bar one Tuesday morning to catch some relaxation after his most recent mission. He ordered nothing to eat or drink, he usually didn't, he only came in to catch the latest rumors and war stories. And today he was going to hear something very unusual. He took his seat at his usual table, his war buddies were already whispering stories to one another.
"Did you hear about the accursed demon fox?" Uraia, a Chunin ninja asked.
"You mean the Nine Tails? Ya, I've heard of it. Why do you ask?" Yomi replied.
"No, not the Nine Tails. No one has seen the Nine Tails since it attacked the Leaf Village. I'm talkin' about the Ten Tails!"
"The Ten Tails"?! Yomi chuckled, "There are only nine Tailed Beasts."
"No, the rumor is Pain-sama fought it out on the southern border...and it had even more chakra than him!!"
"That's immposible," Yomi retorted, "Pain-sama probably killed the beast."
"No again Yomi! It escaped!"
Later that evening on his way home Yomi began to think about the news he had learned at the bar. It troubled him, but he didn't know why. He got home to see his beautiful wife Briana and his newborn son, Tora sitting in the main room.
"Good to see you home dear." Briana said standing to her feet, "Riane and Vosk just brought Ryun over for a visit. He and Tora seem to be getting along great."
Yomi looked in the corner of the room to see his nephew there. The boy was four months older than Tora and yet even though they were cousins, Ryun watched over Tora like a big brother.
"Also two ANBU came by and asked that you stop by Pain-sama's building tomorrow morning, you have a new mission." Briana continued.
Yomi ate dinner and was delighted to hear that Ryun was staying the night. Ryun lived in Konoha so it was hard for the two cousins to be together. Even though Ryun was only a year old and Tora, only 8 months, the two played fairly well together.
Early the next day he left to see Pain and receive his next mission
Part 3: Yomi's Mission- Defend Hidden Rain
Yomi arrived on time to see Pain himself standing behind his desk. His second-in command, Konan was beside him.
"Lord Pain has a new mission for you Yomi. It is an S-ranked assignment." Konan began.
"I am ready to serve." Yomi replied.
"Good to hear," Pain said turning around to face him, "Your assignment is to defend this village from our last threat...the Ten Tailed Fox."
Yomi was stunned, "The..Ten Tails sir?"
"Yes. Is this a problem?" Pain asked sarcastically.
"No my lord." Yomi said kneeling.
"Then you are dismissed."
As Yomi was leaving he began to wonder why Pain didn't tell him where the beast was. His answer arrived that evening.
Part 4: Under attack! The Fox versus Uchiha
That evening, right after dinner a giant earthquake shook Amegakure. Yomi ran outside his home to see the giant ink black fox standing outside the Village gates. Pain's Summoning body was already fighting it. When Yomi arrived the fox used one of it's tails to wrap up Pain and absorb his chakra. Soon after the foxes' red slitted eyes had the Rin'negan in them. With one swipe of it's tail it killed the Summoning Body of Pain. Yomi used the Summoning Jutsu to summon a large salamander and the the two began to fight. Yomi's Salamander slammed the fox's head with its tail allowing Yomi to activate his Sharingan so that he could predict the foxes movements. The fox began releasing chakra into the air to attack with it's Ten Tailed Strong Ball. Yomi told a nearby ANBU to retrieve his nephew and son and bring them here. Just before the fox was able to attack Yomi used the Forbidden Mangekyou Seal, sealing the Tailed Beast within his nephew, Ryun Uchiha. He then completed the seal by sealing the Ten Tailed Strong Ball chakra inside of Tora Uchiha.
No one knows what happened to Yomi Uchiha or his wife after that. But rumor has it that Tora and Ryun are still alive and well...somewhere.