RE: The Naruversity
Nixofcyzerra's theory on when Indra and Ashura migrated to Sasuke and Naruto.
Right. So we know that strictly speaking Sasuke and Naruto aren't direct reincarnations of Indra and Ashura (spelt with an "H" to differentiate from the Asura Path of the Rinnegan,) as otherwise Madara and Hashirama wouldn't have been able to be revived with Edo Tensei, as their souls would have been reborn and not residing in the Pure World.
Instead, the souls of Indra and Ashura migrate (hence the Viz term "transmigrant") and take up residence in Shinobi who share their ideals, influencing and possibly strengthening their minds and chakra. The question is: When did Indra and Ashura take up residence within Sasuke and Naruto?
Figuring out Indra and Sasuke is relatively simple: In chapter 460, Karin senses a change in Sasuke's chakra since his fight with Killer Bee:
and during Naruto's conversation with Hagoromo, we learn that Naruto sensed Indra's presence within Sasuke during their clash at the Samurai bridge, even if he didn't know exactly what it was. If I had to guess, I'd say Indra migrated to Sasuke right after Sasuke decided to go to the 5 Kage Summit and kill them all. Although it wasn't really expanded on very much at the time, this decision may have been at least partially motivated by Sasuke desiring a revolution (i.e, using power as a key to peace,) which he would expand on almost 250 chapters later (having had conversations with Edo Itachi and Edo Hashirama in the interim which seemed to blunt his hatred somewhat.)
As to when Ashura migrated to Naruto, that's a little trickier. My first instinct was to say that it would have been around the time that Naruto rose above the cycle of revenge by not giving into his anger/hatred and choosing to spare Nagato, but on later reflection, Naruto wasn't really demonstrating Ashura's ideals here. Ashura believed that love and cooperation was the key to peace. However, at the climax of the Pain arc, Naruto certainly wanted peace and to carry on Jiraiya's will to help create a world where people would truly understand one another, but he didn't really have any idea of how to go about it. Naruto admits to Nagato that he doesn't know how to find peace, but is determined enough to find a way that his conviction sways Nagato into putting his faith in him.
My best guess as to when Ashura migrated to Naruto would be just after his first conversation with Kushina's chakra echo/ghost/horcrux:
There are two and a half/three reasons for why I think this. The first is that this is the first time we really see "the power of love" in action in the manga. While Hinata confessed her feelings to Naruto earlier in the manga, it was a confession of romantic love, which, (when the recipient of the question in question isn't some poorly written character who realises that amazingly they too had loved the confessor all along) can be quite confusing and conflicting when you don't really know the other person well enough to return their feelings.
* Naruto's transformations into 6-tailed and 8-tailed Version 2 forms were more born out of despair than love having turned to rage/hate.
Kushina's declaration, on the other hand, was an example of less confusing, unconditional parental love, that affected Naruto enough that he was able to completely overwhelm Kurama's hateful will (which was seemingly augmented by by his Negative Emotion Sensing, judging by
this page. This example of love would have given a new level of understanding of the emotion to Naruto (who's not exactly had an abundance of it in the past) and while we don't read Naruto reflecting on his ideals/will having evolved to mirror Ashura's, the way he talks about Kurama
here and to him
here certainly indicates that he understands
The Power of Love.
All right I admit it, that last one was just for fun.
The second reason that I think Ashura's transmigration happened at this point is the Torii Seal. It came out of nowhere (I figured Naruto would just shut the Eight Trigrams Seal again,) the only other person we've seen use Torii Seals is Hashirama (another transmigrant of Ashura,) with his Sage Art: Gate of the Great God, and when the Torii appeared, Kuruma was surprised and reacted like so:
So I figure that Naruto was initially just going to rebuild and close the Eight Trigrams Seal, but, having been "possessed" by Ashura, instinctively upgraded to/used the Torii Seal instead. Kurama obviously either associates Torii with the Rikudo Sennin, or sensed chakra that was associated with Hagoromo, ie., Ashura's. And if it
was the latter, you'd think he would have been less surprised if Ashura's chakra had been inhabiting Naruto for a while, considering that Kurama was completely aware of Dark Naruto's existence, and considered him the real Naruto.
There is one potential problem with the theory for this reason, however, and that's Minato's chakra mode. Would he be able to enter KCM if he still had an Eight Trigrams Seal himself? Or does he have a Torii Seal too? Minato doesn't have anything to do with Ashura.
The third reason I think that Ashura hadn't transmigrated to Naruto until that point was Naruto's reaction to sensing Indra within Sasuke. Naruto was convinced that despite having learnt Senjutsu and the fact that Sasuke was exhausted, he wouldn't have been able to beat him if they had fought. So apparently Indra was enough of an advantage to outweigh Naruto's Sage Mode and Sasuke's own exhaustion. But throughout the ages the two "brother-hosts" have always seemed fairly even. So if Naruto had Ashura at this point, then Naruto should still have had the advantage.
There may be a slight problem with the evidence supporting my reasoning here, though, depending on the translation of the manga. In chapter 671, we see Naruto flashback to this scene in chapter 488:
Where the implication seems to be that Sasuke has
something within him, something dangerous. But in the last panel of chapter 671's flashback to that scene, we have this minor difference that changes the entire meaning:
That "too" implies that Naruto had already sensed Ashura within himself before he sensed Indra within Sasuke. I guess it depends on which translation is more accurate, or if both are and it's a minor retcon.
As for Naruto being convinced that they would both die, perhaps he somehow sensed on some level that he would soon become the next host for Ashura's chakra, or maybe he just wasn't going to back down from their game of "Ninja Chicken."
*Plus Naruto apparently thought Hinata meant she loved him as a comrade. Yeesh.