Transparent World

The Transparent World (きる) is a supernatural ocular ability only accessible to Demon Slayers that have awakened the Demon Slayer Mark.

Description
Demon Slayers that have awakened the Demon Slayer Mark can utilize a special form of ocular extrasensory perception that alters their vision, allowing them to perceive other living beings as if they were transparent, only being able to see the inner workings of their bodies. Most notably, users are able to perceive the flow of blood, breathing, musclular contractions and joint movements of the creature they are looking at. Potentially, one can use the Transparent World's power to see inside an enemies body as a way to predict and evade their opponent's attacks, as well as to find openings for their own strikes.

Additionally, the user is able to perceive the world around them as if it was in slow-motion, thus dramtically increasing their movement speed and reflexes.

Due to the varying circumstances of their awakening, the specifics of how a user accesses the Transparent World is currently unknown. However, one common theme surrounding the accessing of this ability is that the user must focus on looking at their target.

Yoriichi Tsugikuni
As he was born with the Demon Slayer Mark, Yoriichi was constantly accessing the Transparent World ever since he was a child. He first displayed his proficiency in using this ability against his father's subortinate, who was a sword instructor, where child Yoriichi landed four strikes on the swordsman in the blink of an eye, knocking the instructor unconscious and causing swellings the size of fists. After the incident, Michikatsu queried Yoriichi about how he had managed to accomplish such a feat, to which he responded by saying that his opponent's lungs expand right before they strike, and that he must simply observe their bone disposition, muscle contractions and blood flow.

When Yoriichi encountered Muzan Kibutsuji as an adult, the Demon Slayer used the Transparent World to perceive the Demon King's bizzare anatomy and created a new technique to attack and destroy all of his vital organs at once.

Kokushibo
It is unknown when or how he awakened the Transparent World, but Kokushibo (similar to Yoriichi), is able to access the Transparent World constantly without any drawbacks. During the Infinity Castle Arc, Kokushibo first uses the Transparent World to determine Muichiro's identity has his descendent just by glancing at his organs and muscles. Kokushibo also used the Transparent World to perceive Gyomei's bodily structure, and remarked that it was at its peak.

As he is constantly using the Transparent World, Kokushibo can effectively use it to anticipate his opponent's attacks and strike back when they are open.

Tanjuro Kamado
Tanjuro stated that he learned how to access the Transparent World from the father in the form of perception training, and was able to use it proficiently against a giant bear, chopping the animal twice in the blink of an eye before it could even attack.

Tanjiro Kamado
After he was enraged by Daki and went on the offensive, Tanjiro briefly used the Transparent World through unknown means when he perceived Daki's interweaving sashes in slow-motion. During a climactic point in the fight against Hantengu, Tanjiro achieved the Transparent World and saw Hantengu's real body inside the heart of Urami. In the midst of battle against Akaza, Tanjiro remembered his father teaching him about how to access the Transparent World. By remembering the perception training he received when he was a child and Tanjuro's words of sealing off his other senses and turning his mind "invisible", Tanjiro accessed it for a brief moment which helped him dodge Akaza's nearly unavoidable attacks. Afterwards, with the help of the Selfless State, Tanjiro blitzes Akaza and slashes off his arm before the Upper Rank can react, then manages to behead him with Hinokami Kagura: Setting Sun Transformation technique.

During the beginning of his fight against Muzan, Tanjiro was shown accessing the Transparent World and looking at Muzan's vital organs. However due to the excessive amount of damage from Muzan's poison, he wasn't able to stay in that state for more than what seems to be a few minutes. Even afterwards, Tanjiro attempts to use the Transparent World multiple times, but fails mainly due to lack of oxygen caused by his fatigue.

Gyomei Himejima
After witnessing Kokushibo's overpowering attacks, Gyomei pushed himself to the limits and started accessing the Transparent World, being able to form a clear image of the Upper Rank and his organs despite being blind. Gyomei started using the Transparent World offensively by throwing his juzu bead the moment he swung his chain, using the Transparent World to counter the Upper Rank's own so that it slipped through his sight. He used this again by swinging his flail and tearing off a chunk of Kokushibo's upper body without the latter noticing.

It is later revealed that Gyomei not only proficiently used the Transparent World to land attacks on the Upper Rank, but also took his eyes into account and manipulated his own blood flow to throw off his senses and disrupt this attacks.

Muichiro Tokito
After having his arm slashed off by Kokushibo and being injured, Muichiro somehow accessed the Transparent World when he returned to the fray and intuitively used it to land an attack on Kokushibo.

Obanai Iguro
After being instructed by Gyomei, Obanai briefly accessed the Transparent World by focusing on Muzan's body, thus accurately perceiving the Demon King's organs.