Uchiha Sasuke possessed the legendary Rinnegan; but everyone knows he was not born with it.
All the members of the Uchiha Clan are born with gifted eyes; Kekkei Genkai aka Sharingan. These eyes are so powerful that they can trap anyone in despair with just a gaze.
Sharingan also has its evolutions. They can evolve into Mangekyou Sharingan and eventually to the Rinnegan, which possesses godly powers and can rule over life and death itself. Characters like Madara and Nagato have awakened their Rinnegan and you know how strong they are.
So, how did Sasuke gain Rinnegan?
WARNING: Major Spoilers!
Sasuke was not born with Rinnegan
Technically, there was always a possibility that he’d possess Rinnegan as he was born with Sharingan.
But not without a cost.
For Sharingan to evolve any further, one must go through a lot of pain, agony, and suffering. Sasuke took this path, and he encountered darkness, then he started to stray away from the better path of life.
When any member of the Uchiha Clan goes through such emotional outburst that include a lot of pain; they get a special flow of chakra in their eyes. This leads to the evolution into Mangekyo Sharingan; then into Rinnegan.
And ever since the writers introduced Sasuke; fans knew he was powerful enough to get the Mangekyo Sharingan faster than any other member from his clan.
How Did Sasuke get his Mangekyo Sharingan?
For anyone to evolve from into Mangekyo Sharingan; they need to experience extreme pain; like the death of a loved one.
However, Sasuke never had someone he loved, he rather hated his brother Itachi more than anyone in the world for what he did to their clan. But poor Sasuke had no idea that his brother did everything to save the village because Uchiha Clan were planning a rebellion against Konoha.
He loved Sasuke but showed hatred to make his baby brother strong enough.
When Itachi joined with the Akatsuki to collect more information for Konoha; he suffered from a cadaverous illness. Even though he was so weak, he wanted to have the last battle with his brother.
Itachi overpowered Sasuke in the battle, but in the end, he died with a smile in front of his brother. When Sasuke came back to his senses, Obito told the truth about Itachi’s intentions.
This broke Sasuke. The pain of Itachi’s death eventually awakened his Mangekyou Sharingan.
Sharingan can also be implanted from other direct blood relatives like; siblings or parents.
How Can Rinnegan Get Awakened?
Samsara Eye Rinnegan is an extremely rare Dōjutsu, and even in Naruto, we have seen that only a few were able to awaken their Rinnegan.
A person can only awaken Rinnegan by combining the chakra of Asura Otsutsuki and Indra Otsutsuki; sons of Kanna Ōtsutsuki, or they can awaken Rinnegan by directly receiving it from Hagoromo himself.
The legendary leader of the Uchiha Clan, Uchiha Madara also awakened his Rinnegan by taking Hasirama’s blood cells into his body. He mixed Indra’s chakra with a part of Ashua’s chakra.
Thus, he successfully awakened Rinnegan.
So, even if someone awakens their Mangekyo Sharingan, there’s no guarantee that they will awake their Rinnegan.
How Did Sasuke Get The Rinnegan?
Sasuke obtained his Rinnegan while in his subconscious state. Naruto and Sasuke were talking to Hagoromo.
At a particular during their conversation, Hogoromo realized that these two combined had the capabilities to change the Shinobi world and prevent Kaguya from taking over the world.
In the end, Hogormo entrusted Sasuke with his Rinnegan along with the Yin; half of the Six Paths chakra. He gave the another half (Yang) to Naruto to help them seal Kaguya.
Why Does Sasuke Have One Rinnegan?
Sasuke got half of his Six Paths chakra, Yin along with Rinnegan while the other half (Yang) was given to Naruto.
Hogoromo did this because Uchiha Madara; the leader of the Uchiha clan was about to revive Kaguya. There was a desparate need of a way to stop her.
Hogoromo knew Kaguya couldn’t be stopped easily by a single person; the strength of two people was required to seal her.
So, Sasuke only got the half portion of the chakra, Yin. This is the same reason he only awakened one Rinnegan.
But don’t underestimate his powers! Despite having only one Rinnegan; he still had the upper hand in the Shinobi war and showed how powerful he’d become.
Rinnegan showered him with new abilities like; absorbing chakra, casting Chibaku Tensei, gaining access to space-time ninjutsu, controlling the push and pull of gravity, and so much more.
He became near invincible.
Looking back, Hogoromo made the right decision by giving Sasuke only half the chakra of Six Paths. If he had given him the full chakra and both Rinnegan, he might have helped Naruto and the everyone else to seal Kaguya. But after that, no one would to stop Sasuke and he would’ve been the most powerful character in the anime.