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.
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.
How Sasuke Awakened The Mangekyo Sharingan
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 from Kumogakure. Although Bee managed to escape, Sasuke was able to gain control over his eye and figure out what abilities it offered to him.
Sasuke's Left Mangekyo Sharingan
As with nearly all Mangekyo Sharingan, each of Sasuke's eyes grants him a specific power. During his fight against Killer Bee, Sasuke figured his way around his left eye which lets him use the power to use Amaterasu. Sasuke's brother, Itachi Uchiha, was the first user of the Amaterasu in the series. He described this jutsu as the highest form of Fire Release as it allows the user to produce black flames that are inextinguishable until the target is completely incinerated. Itachi was seen using this power extensively in his fight against Sasuke. After awakening his Mangekyo Sharingan, Sasuke was also able to use the same power with his left eye. When used against Killer Bee, Sasuke was able to end the fight in an instant, even though Bee actually survived. With time, his control over these black flames only got better and he was able to coat objects in them and even use them through his other eye.
Sasuke's Right Mangekyo Sharingan
While Sasuke's left eye ability was similar to that of his brothers, his right eye ability is completely different. Using his right eye, Sasuke can use a jutsu called Kagutsuchi. This technique allows him to manipulate the shape and form of the black flames that he produces. As such, Sasuke gains full control over the black flames of Amaterasu, making him an even better user of this power than his brother, Itachi Uchiha.
Kagutsuchi allows him to create weapons such as spears, swords, and even lances with black flames by manipulating their shape according to his will. Furthermore, Kagutsuchi also allows him to use his black flames as a defensive mechanism. This was seen during his fight against the Raikage when he coated his skeletal Susanoo with these flames. Since contact with them can prove deadly, Kagutsuchi becomes an excellent form of both offense and defense. By the end of the series, Kagutsuchi was so powerful that it could even burn through Kaguya Otsutsuki's Ice Release.
Sasuke's Susanoo
Every user of the Mangekyo Sharingan who awakens this power in both eyes is granted the ultimate ability of the Susanoo. This power is known to be the strongest ability of the Mangekyo Sharingan. Upon being used, the Susanoo manifests as a giant humanoid made from the chakra of the user. It offers the highest form of attacking and defensive power to the user and grants tremendous strength. Sasuke first manifested his Susanoo during the Five Kage Summit arc and it only got stronger from there onwards. His Susanoo grants him power great enough to withstand blows from the likes of the Kage and destroy anyone within a matter of seconds. Sasuke's Susanoo is also equipped with a crossbow sometimes and, once completed, it is able to use katanas in combat. Once Sasuke transplanted his brother's eyes into his own, he was able to manifest the Full Body Susanoo, the power of which is great enough to slice mountain ranges apart. Undoubtedly, Sasuke's Mangekyo Sharingan might just be the most overpowered one in the series.