Hiken - Tsubame Gaeshi (秘剣・燕返し?, lit. Secret Sword: Swallow Return or Swallow Reversal, localized as Secret Technique: Turning Swallow Strike) is the fabled technique of the legendary swordsman Sasaki Kojiro who was said to be able to cut down a swallow in mid-flight. He achieved a Mystery attained only with human techniques that rivaled the Noble Phantasms of Servants. It is not something recognized as a Noble Phantasm under the Fuyuki Servant system, but rather an intrinsic Skill of the anonymous swordsman representing and using the legend of the great individual Kojiro as his source of identity. While still unranked, it is classified as a Noble Phantasm under the FATE system. This demonic sword technique, the utmost and sole technique used by Assassin, is not a Noble Phantasm or a magical phenomenon, but it has reached a level comparable to one out of pure godlike skill that "surpasses even Servants." It is a technique where three concurrent arching blades close on the enemy from all sides to create a prison, allowing no chance for defense or evasion. While he downplays it as simply successive slashes done "quickly", it is actually bending the laws of physics to multiply its existence. It starts with a flash from this world, followed by two absolutely simultaneous, over-lapping flashes that disregard the concept of time and space, producing a slash attack that arrives from three different directions by momentarily using the Multi-Dimensional Refraction Phenomenon of Zelretch's Second Magic to transcend speed, dexterity, feints, and many other elements. It is an extremely important secret technique to Assassin, who never takes up a stance, which can be seen by observing how he lifts his sword and swiftly launches the technique.
He has an extremely long range with his 150cm Monohoshi-Zao, which would normally be ineffective at close range where he uses the technique. The first strike comes at the opponent with overwhelming speed, creating a circular arc horizontally to surround them. This isn't much trouble on its own, especially with the length of the blade, but the slightly slower second vertical strike meant to block off escape from above and the third vertical strike to keep the opponent from escaping to the side completely overwhelms the opponent. It is a technique that "challenges god" with mere human skill, making it impossible to block. If one of the slashes is blocked, the other two will hit. Due to the close range, if the opponent attempts to retreat, the longsword will quickly cut them down. Assassin normally fights on higher ground to have an advantage over his opponents, but Tsubame Gaeshi requires a level foundation in order for it to be perfectly executed. If he lacks a proper foothold, he will only be able to make the first two slashes. As this technique is born of prodigious skill instead of a magical ability, it requires no additional magical energy beyond what Assassin needs to move.
Like Gáe Bolg, it is impossible to avoid, albeit through different mechanics. It is possible to develop countermeasures against Gáe Bolg after learning that it targets the heart, but the only true countermeasure against Tsubame Gaeshi is to not allow him to use the skill at all. It is possible to avoid the incomplete version with two slashes by escaping to the side, but the real technique is nearly impossible to survive. Saber, fighting with pure technique due to being unable to use Excalibur at that range, manages to defeat it after Assassin allows his sword to be damaged in order to lure her onto flat ground. Through her precognition and pure determination, she is able to see the weak point in the technique, a tiny opening between his left arm and waist. She only barely manages to avoid being cut while rushing through the opening, and the only reason it was a success was due to her experience with the technique used beforehand on her and the damaged blade. If the blade had been in perfect condition, the technique would have stayed invincible. Fate/unlimited codes features a technique called Hiken: Tsubame Mai (秘剣・燕舞, lit. Secret Sword: Swallow Dance?), which is a series of six blinding strikes around the enemy, followed by Tsubame Gaeshi.
The process behind the development of the technique was simply Assassin attempting to figure out the means of cutting down a swallow in mid-flight. He found that swallows feel the shaking of the wind caused by the sword strike in order to avoid the blade. It is impossible to avoid disturbing the air, no matter how fast the strike, and a sword can only move in a straight line. The swallow can move horizontally or vertically to avoid it, so a single strike cannot possibly cut one down. The swallow's escape route must be completely blocked off, meaning that after the first attack, another is necessary to entrap it. The second blow cannot possibly be done in time to hit the swallow, and it is beyond human capabilities to attack multiple times in one instant. He accepted that it was impossible, but with nothing else to do as a simple and honest reason, he spent his entire lifetime practicing the sword in order to reach the pinnacle of swordsmanship until the very end. After a long period of time, he perfected the technique and it reached another state entirely. Although with some limitations, he eventually attained, perfected and refined his goal, a Mystic Sword technique as a prison of slashes that treads into the Second Magic without any knowledge of Magecraft or True Magic by doing nothing but spending his life swinging his longsword, following a single worthless idea on how to kill a flying swallow.
There is a sword of infinity that leads to multiplicity. A way to acknowledge "multiple possibilities". A single act that is originally finite; a single slash that can only be performed at one time, in one space. It is, so to speak, an "infinite" sword that gives birth to several "correct answers". To make one's own existence transparent to the extremes, till there is nothing more. And yet to grasp for "something" beyond that. That is Kojiro's "Tsubame Gaeshi". A sword that creates a "future" that not even the gods can escape from.
- Fate/Grand Order material II - Sasaki Kojiro, p.006-011
 Fate/Grand Order - Assassin Profile [T]
Kojirou Sasaki - Assassin
Illustrator and Voice actor
Voice Actor: Shinichirō Miki
Eye of the Mind (False): A
Presence Concealment: D
Type: Anti-Unit Mystic Sword
A swordsman described as a worthy rival of one of Japan's prominent master fencer - Miyamoto Musashi.
Mastering the Ganryuu-style at a young age, a genius swordsman said to find no equal no matter where he went.
Level 1 Bond
Source: Historical fact
Alignment: Neutral Evil Gender: Male
"No matter who may be, one is evil from the moment he cuts down a person."
Level 2 Bond
A refined man who admires the beauties of nature and wraps his body in an elegant battle surcoat.
Aloof from the world and having a somewhat elusive personality but, as expected, he is a warrior who likes fair and square contests.
Due to living in a age where casualties happened in a daily basis, his perspectives about life and death may also seem cold-hearted for a modern person.
Level 3 Bond
- Tsubame Gaeshi
Anti-Unit Magic Sword. Maximum Targets: 1 person.
A sword technique that rivals Noble Phantasms, attained only with human techniques.
A special move that simultaneously cuts apart an enemy with three circles.
Seems to be one of the things that can be called multi-dimensional refraction phenomenon.
Originally, it was something developed to cut a swallow flying on the sky.
Level 4 Bond
He himself calls it a secret sword technique, but on the eyes of the surroundings it is unmistakably a magic sword.
For that is not the work of man.
He himself says that it was developed for cutting a flying swallow, but, of course, it is possible to cut a swallow even without a mystic technique such as this.
Rather, this is a story about the lack of compromise, and also, how much extreme talents for martial arts was possessed by this swordsman - who reached the blessings of the sword just for such a reason.
Level 5 Bond
Souwa's Understanding: B
A unique ability that prevents drop on hit-accuracy no matter how much the same technique, the same sword skill is employed against an opponent.
It becomes impossible to see through attacks.
"The eyes grow used to the attacks"; maybe it means that such inferior sword swings are not unleashed.
Unlike Musashi, Sasaki Koujirou is a fictional swordsman whose real life is indefinite.
This "Sasaki Koujirou" is nothing but an nonentity swordsman that was summoned due to the fact he can reproduce the "Tsubame Gaeshi" that appears in Koujirou's legend. The mental state of this genius swordsman - who remained anonymous for life, and was summoned in order to fill up for a fictional Heroic Spirit - is, of course, "that is fine too" it seems.
佐々木小次郎 - アサシン
- Tsubame Gaeshi
 Fate/side material - Encyclopedia: Assassin [Servant], p.055 [T]
Heroic Spirit of assassination.
Different from other Classes, only certain Heroic Spirits qualified for Assassin.
He was the samurai who guarded the mountain gate of Ryuudou Temple.
Sasaki Kojiro was summoned into this class because a certain unorthodox summoner violated the rules.
Strictly speaking, Assassin was a Wraith rather than a Heroic Spirit.
He seemed to be a martial artists who had connections with the Ryuudou Temple in the past.
His true name was unknown. To be more specific, he never had a true name.
Perhaps because he never had the possibility to become a liege, he had no ambitions. He lived only for the wish to stand in the inner sanctum of swordsmanship, and eventually he achieved a Mystery that rivaled the Noble Phantasms of Servants.
In terms of pure swordsmanship, he was the greatest in this Holy Grail War.
Since he was summoned with the mountain gate as the catalyst, he could not leave the gate.
...Huh? Almost like a Jibakurei eh?
 Fate/complete material III: World Material - The Servants of the Fifth Holy Grail War: Assassin, p.034-035 [T]
Alignment: Neutral Evil
Noble Phantasm: ??
Kojiro Sasaki. A Japanese man without a real identity, and it may not be an overstatement to say that he has never existed. He was described as the rival of Nitoryu sword master Miyamoto Musashi, and a handsome swordsman who skillfully wielded the 3-shaku-long (approximately 1m) nodachi “Bizen Osahune Nagamitsu” which is also called “Monohoshi Zao”. His duel with Miyamoto Musashi at Funajima Island, such lines as “Kojiro has been defeated!”, and the finale of death by having his forehead crushed by an oar are all legendary. Although he was said to be the disciple of Chujoryu kodachi master Toda Seigen, there are numerous conflicting documentations and legends, which still shroud his true identity even now.
The “Kojiro Sasaki” summoned in Fate/Stay Night is not a Heroic Spirit. He is an existence closer to wraiths. He was a nameless martial artist given the name Kojiro Sasaki. What his real identity was, there is no way to know. He seemed to be related to the Ryudo Temple. He uses a long Japanese katana and discovered the ultimate technique “Tsubame Gaeshi”... and these are absolutely all he possesses. His pursuit of sword technique, sophistication, and finding joy in duelling with strong foes are all reminiscent of the legendary Kojiro Sasaki: cool and elegant.
Assassin’s differences with Kojiro are that, his katana is longer (approximately 1.5m), and the engraving on the katana is “Bitchu Aoe” (Blue River of Bitchu).
Although a Servant of the Assassin class, he does not employ assassination techniques such as ambush, instead frequently choosing to fight face-to-face. During battle, Assassin counters the opponent’s attack with sword techniques and Eye of the Mind, or simply by dodging. If he detects an opening, he would decapitate the opponent with the 5-shaku katana with certainty. Also, Assassin can deliver slashes even while standing idly, without assuming any stances. His technique is extremely swift, and very difficult to read. Kojiro Sasaki’s ultimate weapon is the technique that has reached the pinnacle of training and approaching True Magic --- Tsubame Gaeshi. It is an attack that delivers 3 slashes simultaneously, enough to overwhelm even Artoria.
Presence Concealment: D
Since he is not a true Servant Assassin, this is not a class ability. It is his intrinsic ability as a martial artist to “cut off his presence”. This is not a skill to hide, but a technique of the mind for the martial artist to merge with his surroundings. Because this is not a skill trained for assassination, it cannot match the Presence Concealment of Hassan. In reality, a Kojiro Sasaki who cuts off his presence is an opponent that is hard to detect even for a Servant. It is extremely difficult to perceive Kojiro during the first encounter. However, because his activity is restrained to within the range of the Ryudo Temple Gate, those who know the existence of Kojiro can detect him relatively easily. With this said, due to Kojiro’s personality, battles are usually commenced only after the parties have presented and introduced themselves. As a result, this ability is rarely used for its original purpose.
Eye of the Mind (False): A
Premonition (“Spidey-sense”) or sixth sense, very different from Emiya’s danger avoidance originated from “predictions augmented by experience”. It may be called a natural talent to sense danger. During the Grail War, in the battle with Artoria, Kojiro crossed blade with an invisible sword numerous times. He was able to use this ability to gauge even the length and width of the sword, and see through the fact that Artoria was unable to use her Noble Phantasm attack due to a lack of mana.
See through the weapon and style of his opponent after crossing blades only a few times, the perception of his eyes is the best among Servants.
An ability commonly called “heart of clear water”. More precisely, it is a mental state acquired after reaching the pinnacle of arduous training. With this ability, Kojiro is able to maintain a cool and collected heart, regardless the situation. For example, even facing his very own death will not cause him to waver. During battle, mental interference such as charm and fear sorceries activated by the opponent is completely neutralised. For a “gate guardian”, this ability is certainly very useful.
Like the many trees that surround the Ryudo Temple, Kojiro watches the gate in silence with a heart of clear water, just like an unwavering guardian.
Knowledge of Respect and Harmony: B
A special ability that prevents the reduction of accuracy regardless how many times the same technique has been used on one foe. In other words, the ability to “render one’s attacks unreadable to the enemy”. Although Kojiro and Artoria crossed blades numerous times, due to this ability, Artoria still could not adapt to Kojiro’s technique despite having Rank A Instinct. Paired with Kojiro’s unique “head-shot” techniques, the effect is vastly increased.
No matter retreating and challenging him how many times, Artoria still cannot see through Kojiro’s techniques. She was always forced back to square one and fought as if it was the first encounter again.
The ultimate sword technique that assaults the opponent from three directions “simultaneously”. Strictly speaking, this is neither a Noble Phantasm nor a magical phenomenon. The nameless martial artist who materialised as “Kojiro” sought nothing but the pinnacle of swordsmanship during his life. He trained himself in pursuit of swordsmanship, and doing nothing but swing his sword. After a long period of time, his technique finally reached another state. A flash from this world, followed by two absolutely simultaneous, over-lapping flashes that disregard the concept of time and space, producing a slash attack that arrives from three different directions. This is the “Multidimensional Refraction Phenomenon—Kischua Zelretch” that transcends speed, dexterity, feint and many other elements. Although with some limitations, a nameless martial artist who knows naught of sorcery or True Magic is able to tread into the realm of the “Second Magic”.
The range of this skill is approximately 3m. In order to deliver the first strike as a horizontal slash, Kojiro relinquished his high ground advantage over Artoria and moved to an equal level before unleashing this technique. You may discern how special a secret technique “Tsubame Gaeshi” is to Kojiro, who never assumes a stance, by observing how he lifts his sword and then swiftly launches the technique.
- Fate/Grand Order Epic of Remnant - Shimosa Parallel World: Chapter 15, Duel between Musashi and Kojiro, translated by anon at /fgog.