Kamui

Kamui is a powerful Mangekyō Sharingan dōjutsu that creates a unique and specialised form of space–time ninjutsu. It allows the user to achieve two very distinct, yet closely associated feats — teleportation and intangibility.

Overview
Kamui allows the user to transfer both objects and living beings to another dimension. Once a target is sent to this dimension it is unable to escape. Obito Uchiha was able to activate Kamui the moment he awakened his Mangekyō Sharingan, while Kakashi Hatake, who awakened his Mangekyō Sharingan at the same moment as Obito without realising, didn't have access to it until sometime after Sasuke Uchiha's defection and made first use of it in the series during his encounter with Deidara.

Both Obito and Kakashi have demonstrated unique uses for Kamui. Using his right eye, Obito is able to make himself "intangible" by transporting portions of his body to the other dimension as they come into contact with other physical objects. Using Obito's left eye, Kakashi is able to teleport anything in his line of sight to the other dimension by creating a barrier space around his target. Referring to Obito's usage, Kakashi likened the characteristics of this technique to those of Minato Namikaze's Flying Thunder God Technique, but noted that it seems to be much more versatile since it requires neither a seal nor a summoning tattoo to be performed.

Teleportation
Both of Obito's eyes are able to transport objects or individuals the user is in contact with to the other dimension. This technique creates a swirling distortion focused on the eye used to activate it, which acts as an attractive force, both visibly drawing the target inwards and distorting their form until they are completely absorbed into the dimensional void and disappear. After absorbing targets through this process, the user is able to eject them from the eye used at any time of their choosing. As demonstrated by Obito's usage, objects can be ejected with varying degrees of force, which allows him to launch projectiles of various proportions and/or quantity with relative ease and at a moment's notice, giving opponents little to no opportunity to react accordingly.

Another unique trait of Kamui is that it allows the user to teleport by absorbing themselves in the same manner that they would absorb another object. The chakra signature of any living being transported through this method, including the user, becomes untraceable. Obito has demonstrated the ability to teleport to any location of his choosing, including the other dimension. Kakashi, who first gained access to this aspect of Kamui during the Fourth Shinobi World War, has only shown the ability to leave Kamui's dimension through this method.

Left Eye
The left eye's primary use of Kamui is based on line of sight and, as such, does not require physical contact to warp a target into the other dimension. By focusing to create a barrier space and concentrating on the target within, Kamui will distort space and the target will be drawn into the other dimension. This usage of Kamui can not only teleport a target in its entirety, but also a specific part of said target, such as an individual's head. At first, Kakashi's aim and control over the size of the barrier was imperfect. However, after gaining skill with the technique, Kakashi was able to use Kamui to quickly transport even small, speeding objects heading toward him. Kakashi can also teleport objects considerably larger than a human, although this requires him to prepare chakra for some time in advance.

Right Eye
The unique trait of this eye is an ability which can be described as intangibility; however, it does not truly make the user intangible. When activated, any part of Obito's body that overlaps with another physical object is seamlessly warped to the other dimension and no longer exists materially outside of it, making it appear as though he is phasing through the object in question. Using this ability, Obito can dodge most attacks, and by overlapping his entire body into large objects, he can not only conceal himself from view, but erase all traces of his chakra. Obito is also able to enter and travel through the ground using this ability, a characteristic he often exploits to ambush his opponents from below. Though he is unable to establish any physical contact while this ability is activated, Obito can still use chakra chains. He also seemingly possesses the capability to extend this intangibility to whatever he's touching at the moment of activation, as long as he is able to maintain some form of contact with at least a part of the desired item or person.

Drawbacks
Since this technique is shared by two different users of the same pair of eyes, both parties can use their own technique to counter the effectiveness of the other, thus making it useless to use against one another directly. Despite this, it is also possible for two users of Kamui to transport each other simultaneously. Even if one user of this technique is successfully absorbed by another, they may escape Kamui's dimension by teleporting themselves out. Obito is also able to completely cancel out Kakashi's usage of the technique, as he negated Kakashi's attempt to decapitate the Demonic Statue of the Outer Path by transporting its head.

Left Eye
The Mangekyō Sharingan is a dōjutsu that, with continuous usage, burdens the user with the loss of their eyesight. Since this is the source of Kamui, using it in rapid succession puts Kakashi's body at risk as well. As his Sharingan was transplanted, usage of this technique poses an even greater risk to Kakashi. Repeated use of this technique puts a great deal of strain on Kakashi's left eye, causing it to bleed if overused.

Kakashi was originally only able to use this technique up to at least three times in a given day, even when starting at full strength. When Kakashi first used this technique, it required great concentration and was difficult to aim, dragging in only Deidara's arm when he aimed for his head. Over time his aim and control improved; by the time of Pain's invasion, he was able to accurately warp away small and fast moving objects, such as a nail being propelled toward his eye. As of the Five Kage Summit, Kakashi was able to consecutively warp away two fast-moving arrows. During the Fourth Shinobi World War, Kakashi demonstrated the ability to use this technique several more times in a single day and also attempted to transport the Demonic Statue of the Outer Path's head, although he required time to prepare the necessary amount of chakra. After being given chakra by Kurama, Kakashi was able to use his long-range variant of Kamui to quickly transport a target as large as Gyūki to Kamui's dimension and later eject the tailed beast from his eye, as well as utilise the technique more times than he would usually be capable of. Kakashi estimated that the influence of Kurama's chakra, when given directly by Kurama, made his usage of Kamui at least three times more powerful than usual.

Right Eye
In Obito's case, the primary weakness of this technique is that both aspects cannot be used concurrently, as he must become and remain tangible in order to transport himself or others, providing a small opportunity in which he is unable to pass through objects and thus, is vulnerable to injury. This deficiency means that he must also materialise when ejecting stored items, presenting the same vulnerability. According to Konan, Obito can only maintain his intangibility for approximately five consecutive minutes at a time, after which he is presumably rendered completely solid for a duration. In order to bypass this weakness and remain in Kamui's dimension for a longer duration, Obito has to "solidify" and absorb his entire body. She also revealed that the time it takes him to absorb something is proportional to its mass, and that it takes longer to absorb himself than another person or object.

Additionally, due to the nature of Obito's intangibility, the sections of his body residing in Kamui's dimension can still interact normally with other objects that are also contained within said dimension, meaning he can be damaged by such items, a characteristic that Kakashi exploited to injure Obito on multiple occasions during the Fourth Shinobi World War. While controlling the Ten-Tails, Obito was unable to extend his intangibility to the beast or his means of connecting to its head, and was separated from it as a result.

Unlike other Mangekyō Sharingan users, Obito's extensive and continuous usage of Kamui does not seem to tire him nor cause his eyesight to deteriorate.

Influence
Kamui is the word for a divine being in Ainu mythology, although it is almost never written with kanji in modern times. In a more general Japanese context, Kamui refers to the might and majesty of the gods, particularly the Shintō Kami.

Trivia

 * The Third Databook seems to hint that other Mangekyō Sharingan users can access this technique through sheer training of the eye, though this is unconfirmed.
 * In the anime, as well as in Naruto 6: Road to Ninja, Obito is shown to be able to access Kamui without having his Mangekyō Sharingan activated, instead using only his regular Sharingan.
 * Occasionally, in the anime, when Obito attempts to absorb an opponent, he does not establish bodily contact with said individual. A notable occasion of this is when he fights Danzō Shimura's bodyguards, as he does not have any physical contact with Fū when he absorbs him.