Kamui

Kamui is a space–time ninjutsu that can be used with Obito Uchiha's Mangekyō Sharingan.

Overview
Kamui allows the user to transfer objects to and from another dimension. These transfers are often characterised by a spiralling void that targets swirl into or out of, distorting their form as they move between dimensions. Targets that enter Kamui's dimension are completely untraceable, as not even their chakra can be detected while they're there. Targets that exit Kamui's dimension do so with varying force, either simply appearing at the ejection point or flying from it; if weapons, such as shuriken, are stored in Kamui's dimension, ejecting them forcefully gives Kamui's users an offensive option.

Using Kamui is the only way to travel to and from its dimension. This means that targets transferred there are trapped, forced to stay until Kamui's user lets them leave; Obito sometimes uses genjutsu on those he imprisons to keep them subdued. This also makes Kamui's dimension effective as a refuge, as anyone taking shelter there can only be reached by using Kamui. But because Obito's two Mangekyō Sharingan spend years with different users – Obito himself and Kakashi Hatake – this makes Obito and Kakashi essentially immune to each other's Kamui: although one of them can use Kamui on the other, the other can just use Kamui on themselves to escape. This also means they can block or counter each other's attempts to use Kamui on others.

Kamui ordinarily only connects its dimension with the Earth. If a user synchronises Kamui with another space–time ninjutsu, Kamui can travel to the other ninjutsu's target dimension. But if the user doesn't have the original ninjutsu to piggyback off of, travelling between dimensions becomes extremely difficult since Kamui doesn't naturally connect to them. Users can manually generate portals to these other dimensions with Kamui, but this requires a great deal of chakra, so much that users will likely need another person's assistance. Additionally, Obito in the Purgatory is able to use Kamui to temporarily return his soul to Earth to continue assisting Kakashi by granting him the use of his Sharingan for a short time.

Right Eye
With Obito's right Mangekyō, a short-range version of Kamui is used. Obito most often uses this version on himself, sucking himself into his right eye to enter Kamui's dimension and then exiting back to whatever location he pleases. Sometimes he does this in order to escape attacks, other times he does this in order to travel long distances. Obito can perform this teleportation at will and doesn't need to designate his destination beforehand, leading others to label it superior to the Flying Thunder God Technique. Others can be sucked into Kamui's dimension through his eye in addition to or instead of Obito, and in fact it is faster for him to use Kamui on others than on himself. Obito is usually shown requiring physical contact with those he's using Kamui on, but this isn't always consistent.

Obito can also use the right Mangekyō to temporarily transfer all or parts of his body to Kamui's dimension. His body (and anything he's holding) will remain visible to others, but they won't be able to interact with him. Obito often uses this ability defensively, transferring his body to Kamui's dimension so that attacks will harmlessly pass through him; during early appearances, while using the "Tobi" persona, Obito commonly pretends to be hurt or feel pain to confuse opponents. Alternatively, Obito can make his entire body intangible, whether to pass through barriers or other obstacles, or to travel underground and surprise opponents with attacks from below. Obito can only keep his body intangible like this for approximately five minutes, and so long as even part of him is intangible he is unable to use Kamui to teleport; in the brief moment between when he solidifies and when he teleports, Obito is vulnerable. Alternatively, opponents can circumvent his intangibility by transferring themselves or their attacks to Kamui's dimension, which will enable them to damage whichever body parts he's currently keeping there.

While Obito is the Ten-Tails' jinchūriki, he is unable to use Kamui to pass through objects.

Left Eye
With Obito's left Mangekyō, a long-range version of Kamui is used. The user looks at a target and creates a "barrier space" around them, with the barrier's size being determined by the user. When the user concentrates, anything within the barrier is pulled towards its centre, transferring the target to Kamui's dimension. When Kakashi first begins using this version of Kamui, he struggles to perform it with any accuracy, frequently missing his target. Even if he does hit his target, he has difficulty making the barrier space large enough to pull the entire target in. In time, he becomes accurate enough to teleport projectiles mid-flight, and later can make the barrier space large enough to teleport the Demonic Statue of the Outer Path's head. Kakashi eventually learns to create the barrier around himself, allowing him to teleport just as Obito does.

Because Kakashi is not an Uchiha, using Kamui demands a great deal of chakra for him to perform. It takes him an extended period of time to gather enough chakra to use it, and once he does he can only use it so many times before his chakra is depleted; originally his limit is three times in a day, but this is later increased to an unspecified number. Similarly, because Kamui is used with a Mangekyō Sharingan, repeated use damages Kakashi's vision, eventually reaching the point where it causes his eyes to bleed. When Kakashi receives chakra from the Nine-Tails, he is able to use Kamui faster, more often, and on targets as large as the Eight-Tails; he estimates that the Nine-Tails' chakra makes Kamui three times more powerful than usual.

Madara Uchiha takes Obito's Mangekyō from Kakashi so that he can gain access to Kamui's dimension. Unlike with Obito, being the Ten-Tails' jinchūriki does not prevent Madara from using Kamui.

Both Eyes
When both eyes' Kamui are used on the same target, whether by a single user or two users working together, the teleportation speed is doubled. Alternatively, two Kamui can be generated separately to achieve a unified purpose: Obito uses the left Kamui to synchronise with Yomotsu Hirasaka, then uses the right Kamui to teleport to its associated dimension. With the two Mangekyō, users can also perform Susanoo, through which they can create Kamui Shuriken.

Trivia

 * 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.
 * Unlike other Mangekyō Sharingan users, Obito's continuous and general usage of Kamui does not seem to deteriorate his eyesight, and likewise does not tire or strain his body.
 * Although teleporting objects from a distance is the left eye's ability, Obito uses the right eye to teleport Kaguya's All-Killing Ash Bones.
 * Occasionally, Obito is depicted using Kamui without activating his Mangekyō Sharingan. Most likely, because it was not yet revealed Obito's technique is the same Kamui Kakashi used.
 * In the anime, the Hizashi Hyūga of Jiraiya Shinobi Handbook: The Tale of Naruto the Hero uses a technique similar to Kamui.
 * The space-time ninjutsu of Shin Uchiha, Nue, and Kinshiki Ōtsutsuki are similar in function to Kamui.