Naruto: Sasuke's Mangekyo Sharingan, Explained

gamerant.com:

The Mangekyo Sharingan is a powerful Kekkei Genkai wielded by only a select few members of the Uchiha clan in Naruto. While nearly all members of the clan possess the Sharingan, which in itself is a mighty power with terrific visual prowess, the Mangekyo Sharingan is very rare. Only those who experience tremendous loss by witnessing a loved one die manage to evolve their Sharingan and turn it into the Mangekyo Sharingan.

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Being the advanced form of the Sharingan, it is a given for this eye to be several times stronger. Itachi Uchiha was the first character in the story to showcase its powers and many others, such as Kakashi Hatake, followed him shortly after. Sasuke, being the most important member of the Uchiha clan in the story, also managed to gain this power and his mastery over it only got better with time.

Sasuke Uchiha knew about the existence of the Mangekyo Sharingan from a very young age. When Itachi slaughtered the entire clan, he challenged Sasuke to awaken this power and stand before him once again. Although Sasuke sought power, he didn't intend to gain it by doing what his brother told him. When the two finally clashed, Sasuke managed to prevail and after learning the truth about Itachi to Obito, he got to contemplate the loss that he had experienced.

After remembering all the incredible times that he spent with his brother and finally feeling his loss in his life, Sasuke awakened the Mangekyo Sharingan. With the awakening of this eye, he gained tremendous power that was great enough to see him fight the absolute top tiers of the Naruto world. Sasuke's Mangekyo Sharingan was first seen in action during his fight against the Killer Bee when he went to retrieve the Eight-Tails