Kenbunshoku Haki

Kenbunshoku Haki, also known as Mantra on Skypiea, is a form of Haki that allows the user to sense the presence of others, even if they are concealed from view or too far to see naturally.

Background
It was first introduced under the name Mantra when Satori used it to fight Sanji, Luffy, and Usopp as they went through the Ordeal of Balls. Aisa is the only known Shandia who has this ability. It served as a plot point when she helped Luffy locate Enel.

It varied in strength between users, with Enel's seemingly being the strongest. With his Devil Fruit, the Goro Goro no Mi, Enel could pick up electromagnetic waves in the air, allowing him to overhear conversations and everything around him; this extended the range of his Mantra to cover roughly all of Skypiea. By comparison, most of the other priests could only keep track of people nearby.

The first time it was seen being used and referred to as Haki was during Luffy's battle on Amazon Lily with the Boa Sisters. Later, Rayleigh explained Haki to Luffy and confirmed that Mantra and Haki are one and the same, and that "Mantra" is simply the Skypiean name for the ability.

Usage
The most common usage of this Haki allows the user to sense the presence of others. Rayleigh states that to fully master this ability would allow the user to see others even if they are concealed from view or too far to see naturally. When viewed this way, the user can't actually see the person with any physical characteristics, but rather recognizes the target(s) by their "aura" which forms as a light silhouette on a dark background, as demonstrated when Usopp sniped Sugar, or how the blind Issho sees the world. This form of Haki is useful in finding "powerful auras" as Sanji displayed in Zou when he sensed Nekomamushi approaching Bege.

The user can also use this Haki to predict an opponent's moves shortly before they make them, thereby making the attack much easier to evade with enough skill. This prediction appears to the user as an image or brief "premonition" of what the opponent will do in the user's mind's eye, and the damage the user will take if the attack actually "hits". It appears that the more killing intent the enemy has, the easier they are to predict, although more skilled users can predict moves whether there are ambient murderous intents or not.

If trained enough, Kenbunshoku Haki users (such as Charlotte Katakuri) can even see slightly into the future. Charlotte Katakuri was able to dodge Busoshoku Haki-imbued attacks using his Logia-like Devil Fruit ability by seeing the attack coming, and making his body flow around it to avoid the strikes. This technique demonstrates that advanced-level Observation Haki is capable of allowing Logia users to defend against Haki-based attacks.

This type of Haki can also allow the user to sense and empathize with the emotions and nature of others. For example, Otohime used this Haki to sense the sufferings and emotions in the heart of a Fishman thief as well as hear the screams coming from Fisher Tiger's heart. Another example is that Aisa was able to sense Wiper'saggressive nature with this Haki. Issho also was able to sense no trace of resentment from the citizens of Dressrosa as they were putting on a facade to help the Straw Hats escape.

It is possible that this form of Haki allows the user to sense the strength of others as well. This was alluded to when Rayleigh used this Haki to sense that there were 500 creatures on Rusukaina who were stronger than Luffy at that time.

When trained highly enough, this type of Haki has been proven to be an effective countermeasure for shapeshifting Devil Fruit users like Logia or Special Paramecia against Busoshoku Haki. With Kenbunshoku Haki, they can predict their opponents' attacks and quickly change the shape of their bodies to avoid contact. An example of this is Charlotte Katakuri who creates holes in his body so the attacks pass through him without touching him.

Kenbunshoku Haki, unlike the other types of Haki, has hardly any visible indication when being used in the manga (like the blackening effect of hardening for Busoshoku or the ripple effect for Haoshoku), aside from their enhanced dodge rate. In the anime special Heart of Gold, anime movie One Piece Film: Z and since Whole Cake Island Arc, those who are able to use Kenbunshoku Haki were shown with bright red pupils when sensing incoming attacks, hidden opponents or seeing slightly into the future.

According to Charlotte Katakuri, Kenbunshoku Haki works best if the user is in a calm state.

Drawbacks
Though the power allows the user to predict most attacks, it can be circumvented by various means. It cannot predict inherently random attacks; for example, Luffy managed to bypass Enel's Mantra by bouncing his fists off a nearby wall, to prevent himself and thus Enel from knowing where they would land.

In the unique circumstance of being swallowed by a larger creature and still being alive, the Mantra user will only be able to sense the larger energy signature. This was shown when Enel failed to discern the presence of Luffy inside the giant snake Nola, only detecting him after he escaped, indicating that her 'voice' most likely enveloped their own when he used the ability. Similarly, Aisa could not detect how the battle was progressing with her Mantra while inside Nola and only found out what happened after she and Luffy escaped.

This power also does not make the user any faster than normal, so the user's ability to dodge is dependent on their speed; Enel was unable to avoid Luffy's finishing move because it was moving too fast, even though he was able to predict it. Likewise, Luffy's Gear Second was able to outpace Boa Sandersonia, despite her ability to read his moves.

This type of Haki is also known to falter if the user loses their concentration. Satori lost control of his Mantra when Luffy unexpectedly brought the string of exploding balls right to him, enabling Luffy to grab him from behind when the smoke cleared, Gedatsu lost to Chopper because he lost his concentration and his Mantra faltered, and Otohime was killed by a gunshot in the confusion when her petition pages were being burned, as she was caught off guard due to her happiness over everyone signing her petitions, she lost her concentration.

The ability to see into the future, which requires an extremely high level of skill, also has its limitations. Charlotte Katakuri was able to see Vinsmoke Sanji dodging another person's gunshot and tried to intervene immediately by attacking Sanji himself. However, he was unable to see the outcome of his own intervention of this event in time, resulting in Sanji surprising him when he dodged his attack as well.

In addition, the user can only interpret the future through their own senses. Katakuri foresaw his sister Pudding crying on the altar, but because he was too far away from her in his vision to hear what had happened, he had no way to know what was about to cause this.

It is revealed that like Busoshoku Haki, Kenbunshoku Haki can be depleted if used too much.

Trivia

 * The concept of Kenbunshoku Haki, known to Skypieans as Mantra, is loosely based on the style of thinking in Hinduism, which goes by the same name.