When a user awakens the Mangekyou Sharingan, they automatically gain Susano'o (provided that they awakened Mangekyou Sharingan in both eyes and not just one) and additional Mangekyou Sharingan abilities unique to them. Generally, it seems users gain 1-2 new Mangekyou jutsu. There is the possibility that a user can get more than 2 Mangekyou abilities with their Mangekyou eyes, but since that's never been shown or explained, I assume that two is the maxim number of Mangekyou abilities per user.
Madara stands as the unlucky exception, as his Mangkeyou Sharingan didn't appear to have any abilities unique to him, at least not any that he used. It's possible that his ability is a very difficult one to discover, so maybe Madara was just never able to figure out his Mangekyou's ability. Or, for an alternative theory, maybe Madara's Mangekyou ability isn't very helpful or handy and thus virtually useless in a fight. After all, the Mangekyou abilities are awakened at random, not something one gains from training, so you don't know what you're going to get and it stands to reason that not every Mangekyou power awakened would be especially useful. Or they might only be useful in very specific scenarios.
Anyway, the Susano'o operates on a different set of rules than the other Mangekyou abilities, so all discussion regarding Susano'o will be omitted from discussion for the rest of this chapter.
With the exception of the Susano'o, an Uchiha's Mangekyou abilities are transferable, meaning they aren't exclusive to the user, so they can be used by anyone who possess the Mangekyou eyes with that particular ability. Meaning that if anyone should steal a Mangekyou Sharingan and take those eyes for themselves, they can use the Mangekyou abilities that those particular eyes have, whatever it may be.
That's why Kakashi and Madara were able to use Obito's Kamui, because anyone who is in possession of Obito's eyes and skilled enough to control the Sharingan can access Obito's Kamui powers. For the same reason, if ever Obito lost both of his eyes, then he wouldn't be able to use Kamui anymore. Rather, he would lose the ability unless he was able to get his eyes back.
Oops! This image does not follow our content guidelines. To continue publishing, please remove it or upload a different image.
Oops! This image does not follow our content guidelines. To continue publishing, please remove it or upload a different image.
The same is true for the other eyes, which is also why Itachi was able to use Shisui's Kotoamatsukami when he had Shisui's eye.
This should mean that awakening the Eternal Mangekyou Sharingan means that a user gets both their own Mangekyou abilities plus the abilities of the one they took the Eternal Mangekyou Sharingan from. So just as the two separate Mangekyou patterns combine into one, so too should the eyes of both users combine into one eye, so an EMS should have 2-4 abilities instead of the Mangekyou's 1-2 abilities, depending on the user and the abilities of the eyes he transplants into himself.
