Awakening the Eternal Mangekyou Sharingan has a pretty straightforward requirement: a user who has already previously awakened the Mangekyou Sharingan has to transplant another user's Mangekyou eyes into themselves. Transplanting the Mangekyou Sharingan triggers a reaction that evolves the eye to its next evolution, dubbed the Eternal Mangekyou Sharingan.
This reaction triggers a change in the pattern design, as the user's unique Mangekyou Sharingan pattern changes to combine the two patterns.
When Madara transplanted Izuna's eyes into himself, Madara's Mangekyou pattern changed to include the distinctive pillars of Izuna's Mangekyou design.
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When Sasuke transplanted his brother's eyes into himself, his Mangekyou pattern likewise changed to include a three-pronged shuriken shape similar to his older brother's three-pronged Mangekyou pattern.
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It is stated that a Mangekyou can only be awakened by swapping the eyes of a close relative, like a sibling or parent. Whether this is true or not is undetermined, but we've only seen blood brothers swap eyes, so I assume this rule is true and that an Uchiha with the Mangekyou Sharingan must have a close relative swap eyes with them to awaken Eternal Mangekyou Sharingan. Thus simply transplanting the Mangekyou of any random unrelated Uchiha member won't work. But little can be used to prove this, since only Madara and Izuna and Itachi and Sasuke have swapped eyes, so that theory remains untested.
It is unclear if two different Mangekyou Sharingan users who are close relatives can just trade their eyes. For example, it seems like it would have been smart for Izuna and Madara to swap eyes at some point to upgrade their Mangekyou to the EMS, yet they didn't do so. It's unclear if this has ever even been tested. It seems likely that the Uchiha would have experimented with it at some points, but if so, we don't know what the results of such tests were.
Itachi mentioned that in the past, Uchiha clan members often stole the eyes of their sibling and Itachi made it sound like it was a morbid detail of the Uchiha clan's dark history.
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However, it must be noted that everything Itachi says about history must be viewed with a healthy dose of skepticism and suspicion, since we already know some of his "historical" information about Madara stealing Izuna's eyes was faulty and incorrect, whether because Itachi was purposefully lying or simply just misinformed. After all, Itachi contradicts himself within his own narrative, as he claims that Madara and Izuna were the first-ever Uchiha to awaken the Mangekyou Sharingan, which I don't believe for a second. Even the anime filler already shows that historically, many other clan members possessed the Mangekyou Sharingan, so it's an old, not new, phenomenon, and many Uchiha prior to Madara and Izuna had awakened the Mangekyou.
But even if Madara and Izuna were the first Uchiha to awaken the Mangekyou Sharingan, then how could the secrets to awakening the Eternal Mangekyou Sharingan be "gradually discovered over time after many, many sacrifices" if Madara and Izuna were the first to awaken the Mangekyou? Something about Itachi's information doesn't add up, so it's hard to discern clearly what is truth and what isn't. Maybe the Uchiha aggressively experimented to understand how to awaken the Eternal Mangekyou Sharingan, maybe they didn't. I highly doubt Madara and Izuna were the first to awaken the Mangekyou Sharingan though, so I can say with certainty that I think that much of Itachi's information is false.
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The Eternal Mangekyou retains the same powers as the Mangekyou Sharingan, although unlike the Mangekyou Sharingan, which causes a loser to go blind over time as the eyes lose their light, the EMS's vision always remains clear and perfect.
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I also suspect the Eternal Mangekyou Sharingan is better suited and adapted to a user's body, thus reducing the amount of pain that it generally requires to operate Mangekyou abilities such as the Susano'o, which is evidently excruciatingly painful to use, or other abilities such as the Amaterasu, which puts an extreme amount of pressure and stress on the eyes.
