The Mangekyō Sharingan is an advanced form of the Sharingan that has only been activated by a handful of Uchiha. They are noted to be the "heavenly eyes that see the truth of all of creation without obstruction".
Itachi Uchiha awakened his Mangekyō Sharingan after witnessing the suicide of his best friend, Shisui Uchiha, shortly before the Uchiha Clan Downfall. With his left Mangekyō he can use Tsukuyomi, an extremely powerful genjutsu that distorts the victim's perception of time. With his right Mangekyō he can use Amaterasu, creating inextinguishable black flames at the user's focal point. Since he awakened both his Mangekyō, he can use Susanoo. Because both Tsukuyomi and Amaterasu use vast amounts of chakra, multiple use of the Mangekyō's abilities in a short period of time is particularly debilitating for Itachi, such that after using it three times in one day, he was required to deactivate even his normal Sharingan to conserve energy. Some time after Itachi's death, desiring the power to kill Naruto, Sasuke Uchiha had Itachi's eyes transplanted into him.
Obito Uchiha awakened his Mangekyō Sharingan after the death of Rin Nohara during the Third Shinobi World War; Obito himself only had his right eye at the time, but the event caused his Mangekyō to simultaneously awaken in Kakashi Hatake, who wielded Obito's left eye. Both eyes have access toKamui, able to teleport targets to and from Kamui's Dimension though in different manners. The right eye is able to use close-range Kamui, teleporting the user or those they're in contact with. By teleporting only parts of their body there, they can in effect seamlessly pass through other matter. The left eye is able to use long-range Kamui, creating a barrier around a target that teleports them. The left eye can also target the user themselves. During the Fourth Shinobi World War, it was discovered that each eye is impervious to the others' Kamui.
Madara Uchiha steals Kakashi's eye in order to pursue Obito to Kamui's dimension. There, he exchanges the eye for Obito's Rinnegan, returning both eyes to their original owner, who was currently possessed by Black Zetsu. During this brief period, Black Zetsu also gained access to both Sharingan powers. After Obito was freed from Black Zetsu's control, he uses them in combination effectively thereafter, but the eyes are destroyed when he is killed by Kaguya Ōtsutsuki's All-Killing Ash Bones. His spirit briefly returns to the living plane and inhabits Kakashi, giving him both Mangekyō's with their respective abilities and, by extension, Susanoo.
Madara Uchiha was the first Uchiha to awaken the Mangekyō Sharingan. It's not known what the individual abilities of each of his eyes were, but he was able to use Susanoo. The straight tomoe of his Mangekyō Sharingan granted him fluidity in his movements when fighting. When Madara's over-usage of his Mangekyō during the Warring States Period left him blind, this prompted his brother, Izuna Uchiha, to bequeath Madara his eyes on his deathbed for the sake of preserving the Uchiha, thus granting Madara the Eternal Mangekyō Sharingan.
Towards the end of his life, Madara was able to evolve his eyes into Rinnegan, which he transplanted to Nagato when Nagato was a child without anybody ever knowing. The eyes eventually found their way back to Madara after he is revived.
Izuna Uchiha awakened his Mangekyō Sharingan shortly after Madara did. It's not known what the individual abilities of each of his eyes were. Before he died, Izuna entrusted his eyes to Madara in order to restore his vision and thus protect the Uchiha from extinction. Despite Izuna giving the eyes willingly, Madara is popularly believed to have taken the eyes against Izuna's will. It should also be noted that Izuna's death allowed his Son, Zaskuare, to Awaken the Mangekyō Sharingan.
Sasuke Uchiha awakened his Mangekyō Sharingan after the death of his older brother, Itachi Uchiha. With his left Mangekyō he can cast Amaterasu. With his right Mangekyō he can shape the flames or extinguish them. Because he awakened both of his Mangekyō, he can use Susanoo. The straight tomoe of his Mangekyō Sharingan grant him fluidity in his movements when fighting. Sasuke is initially reluctant to transplant Itachi's eyes and gain the Eternal Mangekyō Sharingan, claiming that his views were different from Itachi's. Later, he decided he needs the increase of strength in order to kill Naruto Uzumaki, along with the severe deterioration of his eyesight from overuse, Sasuke accepted Itachi's eyes transplant.
After acquiring the half of Hagoromo Ōtsutsuki's chakra, Sasuke's left Eternal Mangekyō Sharingan made way for the Rinnegan and remained visible as six tomoe, three on each of the two innermost circles of the eye.
Shisui Uchiha awakened his Mangekyō Sharingan after he "allowed his friend and rival to die". With each of his eyes he could use Kotoamatsukami, a powerful genjutsu that manipulates a target's thoughts completely without anyone, including the target, having the slightest idea that they were being controlled. However, this technique can't be used with the same eye again until after a prolonged period of time. In the anime, awakening the Mangekyō Sharingan in both eyes allowed him to use Susanoo.
When Shisui attempted to use Kotoamatsukami in order to avert the Uchiha Clan Downfall, his right Mangekyō was stolen by Danzō Shimura, who felt Shisui's plan was futile and wished to protect the village in his own manner. Fearful that Danzō would take his other eye too, Shisui entrusted it to his best friend, Itachi Uchiha, and begged him to use it to save Konoha. Danzō transplants the eye he took into himself and uses it over the years to manipulate others to his way of thinking, using the chakra of Hashirama Senjuto reduce its recharge time. Danzō destroys the eye before his death so that nobody else could have it. Itachi transplanted the eye he was given into a crow that he planned to use to re-ally his brother, Sasuke, with Konoha. He used it on himself instead when he is reincarnated in order to break free of Kabuto Yakushi's control. Itachi incinerates the crow afterwards so that nobody else could have the eye.
Shisui Hashirama Uchiha II
Shisui Uchiha II awakened his Mangekyō when he witnessed the Brutal Beating of his Teammate and Girlfriend, Hinata Hyūga at the Hands of her Cousin Neji's, Cruel Hand's. He would Gain ascess to the Abilities of Kamui and Tsukuyomi. He would only be able to use Short-Range Kamui, and this was believed to be an Isolated Case of Skipping an Eye. However his Son Minato Uchiha, would be able to only use Long-Range Kamui, which would cancel his Father's out, during their attempts to prevent the kidnapping of Sakura Haruno by the Hands of Shin Uchiha. It is unknown why, but this hasn't repeated itself. Later on Shisui's Right eye Evolved into a Rinnegan after Receiving Half of Hagoromo Otsutsuki's Chakra, thus granting him Two Rinnegan.
In the manga, Indra Ōtsutsuki was the first user of Mangekyō Sharingan. The individual abilities of each of his eyes were unknown, except that he was able to use Susanoo. In the anime, he awakened his Mangekyō Sharingan after killing his own two followers. He was also able to utilise Amaterasu with his left eye.
In the anime, Hagoromo awakened this power after risking his brother Hamura's life to save him from Kaguya's control; however, the design of his Mangekyō Sharingan is unknown because his eyes evolved into the Rinnegan at the same time. While the individual techniques his eyes had are unknown, he was able to create a massive Complete Body — Susanoo, which was roughly the size of the Ten-Tails.
This Mangekyō Sharingan form is uniquely shared between multiple (yet connected) individuals. With his right eye, Shin Uchiha is able to telekinetically manipulate any objects after first branding them with a special mark. Shin's clones, being genetically identical to him, possess the same Mangekyō Sharingan design and with it the same telekinetic powers. A creature also shares the same Mangekyō Sharingan design with Shin.
In the anime, Fugaku Uchiha had awakened his Mangekyō Sharingan after witnessing a sacrifice of his dear friend during the Third Shinobi World War. Although, not much is known about his Mangekyō, Fugaku was confident it was strong enough to control the Nine-Tails.
Three days after the Death of his Father Zaskuare Uchiha awakened his Mangekyō Sharingan and gained access to Tsukuyomi and Kotoamatsukami respectively. He did however Obtain the Rinnegan shortly after, as he had a mixed Heritage of Senju and Uchiha Ancestry. This was incomplete however, as his Ancestry was diluted, as a result of the fact his Mother was the Younger Sister of Hashirama Senju, Asura's Reincarnation, whereas his Father was the Younger Brother of Madara Uchiha, Indra's Reincarnation. His skill with his Mangekyō's abilities were so Great, that he came to be known as Konoha's Red-Eyed Raven Flash.
Minato Uchiha Awakened his Mangekyō Sharingan upon seeing his classmate Sumire Kakei Lifeless following an orchestrated attack by Sumire herself. He gained ascess to Long-Range Kamui and Amaterasu. He eventually managed to Use Susanoo, but not to any great extent untill his Training with the Past and Current Raikage's. He eventually killed two of Shin Uchiha's Clones and took their Mangekyō Sharingan in an effort to Gain the Eternal Mangekyō Sharingan, to reduce the Eye Irritation and Blurred Vision from his use of the Mangekyō's Abilities. He would eventually Attain the Rinnegan after Receiving Chakra from the Spirit of Indra Otsutsuki.
Minato's Eternal Mangekyō, would Inherit his own already unlocked Kamui and Amaterasu and Shin Uchiha's would Grant him ascess to the Object Manipulation Technique and Later on Tsukuyomi and Kotoamatsukami. This was strange in that he already had one half of his Father's Kamui Ability, and he now had two Genjutsu-Based Mangekyō Abilities in a Single Eye. He would eventually learn how to combine the Amaterasu with the Rasengan and later Rasenshuriken.
Sarada Uchiha awakened her Mangekyō Sharingan after she believed Minato Uchiha, her then Boyfriend, had been killed after he took a nearly fatal injury and became temporarily immobile. Her Use of the Mangekyō's abilities were not as proficient as her Father's, or Minato's. However they had both had time to practice and train in its use beforehand. It would be years before Sarada began to learn how to truly use her Mangekyō's abilities. What is known is that she could Use Tsukuyomi with her Left Eye.
Problems with Extended Usage
Prolonged Use of the Mangekyō's abilities, especially the Susanoo, is detrimental to the user's Eyesight, and potentially their body depending on their Heritage. Sasuke Uchiha has been quoted to say that his Use of Susanoo, made him feel as though every Cell and Fibre of his Being was on Fire. Use of the Mangekyō's the can also result in the Eye, using the Desired Ability, to Bleed. This can be Reduced to some degree, but only for those who naturally possess either A bit of Senju or Uzumaki Ancestry. Due to Both the Uzumaki and Senju Clan's Immensley Strong Life-Forces and Vitality, this Drawback is drastically reduced, as The Senju and Uchiha are Technically Two Half's of Hagoromo Otsutsuki's Chakra, meaning that it's true strength is unreachable until both Halves are reunited.
Furthermore The Mangekyō's Use is Far more taxing than a Standard Sharingan in any of its three Stages, as continued use will result in eventual Blindness. This can be negated however through a Transplant Surgery. Two Mangekyō's will be combined via a different pair being transplanted into the eye sockets. The Resulting Effect, will restore Eyesight, even from use of Izanagi and Izanami, and will inherit a Piece of Both Eyes Designs. The Eternal Mangekyō Sharingan will never loose their sight, even when using Izanagi and Izanami. However they are still prone to Eye Irritation and occasionally Blurred Vision from using Mangekyō Abilities.
Regardless of the persons Heritage The Eternal Mangekyō and the Mangekyō Sharingan on it's own, still with its issue's.