After Indra and Ashura disappeared from their view, Gamamaru asked Hagoromo, "What is it that you want to test?". Hagoromo replied, "It's not a test or anything. The elder excels, but due to his power, he does not have faith in love. The younger one doesn't have the skill but overflows with love.
Ninshu will come to exert enormous influence in this world. For Ninshu to take a true and proper path, both power and love are necessary. I want my sons to grow during their journeys and learn to nurture Ninshu together".
While Indra went on his own, Ashura was joined by his friend whom he broke out of the prison before and they journeyed together to the destination. When they reached the village outskirts, at first they were misunderstood as bandits were attacked, but after Ashura healed a girl and said they are from the village of ninshu, they were welcomed.
A few weeks later,
Indra returned very quickly and explained how he solved it. Everyone expected Hagoromo to name him as successor but instead, he said, "I will wait for Ashura and will decide it after he returns", which made Indra disappointed.
One year later, Ashura returned with a few others and explained to his father how everyone helped him digging a new well for the land, how they burned it together and healed the people making Hagoromo remember how he built the bridge with everyone at the start of his journey to spread Ninshu.
Hagoromo looked at Ashura "so the Ninshu that you spread has saved many people". Hagoromo turned towards everyone who gathered there "I will now name the successor to Ninshu. The one who will take over the Ninshu will be Ashura". Both Ashura and Indra were surprised by his decision.
Indra who was dissatisfied asked him, "please tell me the reason why I wasn't named as the successor. I did the same thing as Ashura did. The only different thing was that I did it alone and finished it quickly". Hagoromo then replied "do you know what happened after that?. The villagers fought over the rights of water and ended up destroying themselves.
But Ashura labored up a path alongside the people to open up a path. Hard-earned happiness is difficult to destroy. Indra, you have a wonderful power known as Sharingan. Ashura has matured but still lacks in power.
Lend him your power going forward and support the growth of Ninshu". Indra who was frustrated stood up and left that place without replying.
Later, The scene changed to Indra returning to the village with Susanoo after awakening Mangekyou Sharingan and attacked the village. Ashura stood up to him but was completely overwhelmed by his strength. Even so, he didn't give up and continue to battle him. Upon seeing it, for some reason, Hagoromo told everyone to join forces.
When it wasn't enough, Hagoromo spoke: "everyone, lend Ashura your power". After gaining six paths sage mode with the help of everyone, Ashura defeated Indra.
Indra looked hatefully at Ashura "I will never acknowledge your Ninshu or you" and he disappeared from there. Shuichi was back to the spirit world asked "why didn't you stopped your son? You stopped even Juubi".
Hagoromo then said: "I will come back to that part later. Anyway, later, I heard rumors that Indra established a sect of Ninjutsu that idealized power.
I recorded the misfortune that the sharingan brings about in the stone tablet but Zetsu took advantage of it and seems to have been a total waste".
Shuichi instantly interrupted "wait a second. Zetsu? where did he come into the picture?". Hagoromo stared at him for a while "Ah! I guess, I forgot to mention him. You see it's been a thousand years after all. sorry about leaving our a crucial thing.
When I was alive, I know nothing about him, I only found out about him after I died while watching over my sons all this while".
Shuichi frowned "so, Zetsu isn't Madara's will right?". He nodded "He's the will of Kaguya, our Mother. He is the dark force I mentioned earlier who manipulated my elder son Indra's mind and the very reason the hate existed between Senju and Uchiha". While Shuichi was shocked to hear it, Hagoromo continued his story while the scene around him changed. Hagoromo on his death bed spoke, "It seems my time in this world is coming to the end. I wasn't able to guide this world well". Ashura replied, "Father, don't say that".
later, that night, Indra appears before him. hagoromo asked, "have you come to kill me?". Indra replied, "there's no need for me to do that anymore. I just came here to end the future of this hell disguised as Ninshu.
You have always feared the power of my grandmother and instead of trying to understand this enormous power, you merely ran away from it using love and bonds as a shield. Your selfish misconception will mar the future of Ninshu with wars and calamities.
Until I destroy Ashura and his brethren and wipe out Ninshu from the face of this world, I have no intention of stopping this fight. Until I accomplish this, my soul will reincarnate and fight however long it takes!".
then the scene changed to Hagoromo grabbing Ashura's hands "forgive me". Ashura replied "Father, be at peace. If Brother's soul will reincarnate over and over again. So, shall mine". Hagoromo looked at him and smiled "Ashura, I entrust the future of this world to you and your descendants".
He died later while thinking "did I do the wrong thing back then? If only I stopped him when he went with Hamura, would the outcome be different from how it is now?. I was able to bring peace to the entire world but wasn't able to do anything to my family".
Back to the spirit world, Hagoromo told Shuichi "just like how Indra warned, the battle between them waged on, never-ending, over and over again. These battles took the lives of many comrades and kin on both sides.
The fighting continued even after Ashura and Indra died to Hashirama Senju and Madara Uchiha and it still dragged on now to the current generation Naruto Uzumaki and Sasuke Uchiha".
Shuichi was shocked "what!! Naruto and Sasuke were your son's reincarnations? First Hokage and Madara too?. I can understand Naruto as I found both of them were very similar but Sasuke? seriously? I mean, he seems to be a little too weak compared to Indra".
Hagoromo shook his head "that's because you view both of them from a different angle. But anyway, I only wanted to tell you briefly but I think I was swept up in it and took too much of the time without explaining your question".
Shuichi shook his head "no, I got my answer. It was to recreate your chakra, isn't it? since Madara was Indra's reincarnation and Hashirama was Ashura's. But, it raised one more doubt for me". Hamura replied instead of hagoromo "I know what you want to ask. How did you awaken Rinnegan, right?". Shuichi nodded.
When Hagoromo opened his mouth, Hamura tried to stop him "Brother". He looked at Hamura "it's alright. he has the right to know". While Shuichi confused for his hesitation, Hagoromo looked into his eyes "it's because, from the start, you have both of our chakras. It was just dormant as you were lacking a necessary component.
It was woken up after you were injected Hashirama's cells and becasue of the trauma caused by your sister's death, you were able to awaken the Rinnegan completely". Shuichi was shocked that his body shivered slightly for a fraction of a second "What did you just say?. Are you implying that I'm your reincarnation?"
He shook his head "no". Shuichi replied, "as expected". Hagoromo replied with sadness "you are the reincarnation of Teiji Otsutsuki, my son or should specifically say my only human creation who is born from Mine and Hamura's chakra".
Shuichi was stunned for a while and went into an extremely shocked state. he slowly spoke "say it once again? did you just say that I'm the reincarnation of your experiment?".
basically he's like a tailed beast born from both of their chakras. Originally I want to make him son of Hamura.
But it sounds cliche for me. so I made him a human that just born through the creation of all things jutsu instead of the natural way in order to give him Rinne tenseigan later.