As per usual, got bored and couldn't be bothered to do the relevant jutsu as planned.
Mangekyo Sharingan v1 (
Version 2)
The Mangekyo Sharingan is the evolved state of the Sharingan, the kekkei genkai of the Uchiha Clan. The three tomoe transform into a new pattern unique to the individual, hence why it is known as a 'kaleidoscope'.
The evolution increases the innate power of the Sharingan itself and it is able to directly amplify the Eye of Insight to improve the clarity of its perception or the Eye of Hypnosis to weave even more powerful illusions. Those that excel in amplifying the former are known as Straight Tomoe and those that excel in amplifying the latter are Cursed Tomoe.
In addition to strengthening the Sharingan's existing abilities, the Mangekyo allows its wielder to harness at least one powerful jutsu that makes them far more dangerous than the typical Uchiha. These abilities would take years, if not decades, for ordinary ninja to create and likely still would not be as powerful as the Mangekyo's.
And lastly, those with the Mangekyo Sharingan possess the ability to dominate the Tailed Beasts and bind them to their will. The awakening causes their chakra to reach a state closer to its original source, a connection which gives it power over them.
The cost of the Mangekyo Sharingan is that its eyesight inevitably degrades until renders the Uchiha blind. Even if they never use the eyes, they would eventually lose their eyesight but the more they use the eyes then the faster they go blind.
Each use of the Mangekyo Sharingan costs a significant amount of chakra and injures the eye itself and makes it bleed. The combination of pain and fatigue makes it very difficult to wield the doujutsu for prolonged periods in battle, and the more they use it in succession then the faster the damage progresses.
Conventional medicine and medical ninjutsu are not able to restore any of the lost eyesight. At best, they can delay how quickly the eyes go blind but it cannot reverse the damage itself.
Very few have ever awakened this power yet its origins lie with the progenitor of the clan.
Otsutsuki Indra had the Sharingan and its powers of insight and hypnosis from birth but he harnessed even greater ocular powers later in life. He found that he could transform his eyes into the Mangekyo Sharingan - though he did not have such a name for it - through which he can unleash various powers such an igniting an unquenchable flame or weaving an unbreakable illusions. These abilities were passed down to his descendants but was buried deep in their bloodline and requires an extreme trigger to draw them out.
While the Sharingan is awakened through strong emotion, this is awakened by directly experiencing the traumatic loss of a loved one and granting them even greater power.
The first to awaken the Mangekyo Sharingan were the brothers Uchiha Madara and Uchiha Izuna. Their father had been steadily losing his mind for years due to their bloody lifestyle and his instability fractured the clan yet his strength kept him in charge. To save their clan, the brothers killed their own father. However their love for their father made it a painful deed which acted as a powerful emotional trigger for the Mangkeyo Sharingan. They discovered this new power together and used it to unite the clan again behind Madara as their new leader.
Eternal Mangekyo Sharingan
The Eternal Mangekyo Sharingan is the final stage of the doujutsu.
By replacing their Mangekyo Sharingan with that of a direct blood relative such as a sibling, parent or offspring, the host's chakra mixes with the donor's in the transplanted eyes to awaken a more powerful version of the doujutsu.
Those who possess it can wield all of the powers of their Mangekyo Sharingan without suffering any physical strain or blindness. This evolution is reflected in the changed pattern of the eyes which are a combination of the host and donor's original ones.
The doujutsu's abilities are actually no more powerful than before. Every use of the eyes in their previous state was physically painful to use and had permanent consequences meant that it needed to be used sparingly; they would almost never be practiced like ordinary jutsu and would be tested in live combat instead. Now that the only cost is the amount of chakra, the wielder can practice and experiment with their doujutsu as much as they need and their concentration isn't disturbed by the pain. This makes it far easier for them to draw out their doujutsu's true potential and makes a Uchiha with the Eternal Mangekyo Sharingan far more dangerous than their previous self.
The only two people to have ever awakened the Eternal Mangekyo Sharingan are Uchiha Madara and Uchiha Sasuke.
In his numerous battles against Senju Clan, Madara had nearly lost all of his eyesight from overusing the Mangekyo Sharingan. As his younger brother Izuna lay dying from their last battle with the Senju, Izuna offered his less damaged eyes to his brother so that he could continue protecting the Uchiha Clan in his absence. Madara tore out his own eyes and replaced them with his brother's, unknowingly starting the transformation that restored its sight and made it eternal.
Those who followed in the brothers' footsteps and awakened the Mangekyo Sharingan themselves tried to achieve this evolution as well but continually failed. With there only being a single precedent - and the exact circumstances involved weren't something that Madara shared with his brethren - Uchiha would steal the eyes of their own clansmen in the hopes of awakening the Eternal Mangekyo Sharingan only to fail continually. It was only through trial and error that they came to the conclusion that it would only work with a direct blood relative who had awakened the Mangkeyo Sharingan themselves.
But awakening the Mangekyo Sharingan became less common over the generations; the initial craze for its power fading and the Uchiha placing strict control over one another. It became a rare thing but those who did possess its power were still respected among the clan for said power.