Tsubame Gaeshi

Hiken: Tsubame Gaeshi (秘剣・燕返し) is the fabled technique of the legendary swordsman, Sasaki Kojirou, who was said to be able to cut down a swallow in mid-flight. This demonic sword technique, the utmost and sole technique used by an anonymous fictional swordsman created from the various legends of Kojirou, is not a Noble Phantasm or a magical phenomenon, but it has reached the level of 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, 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 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 Monohoshizao, which would normally be ineffective at the close range where he uses the technique. The first strike comes at the opponent with overwhelming speed, creating a circular arc 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 horizontal strike to keep the opponent from escaping to the side completely overwhelm 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 prana beyond what Assassin needs to move.

Like Gae 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 a 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 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), 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, he spent his entire lifetime practicing the sword in order to reach the pinnacle of swordsmanship. After a long period of time, he perfected the technique and it reached another state entirely. Although with some limitations, he eventually perfected and refined 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 sword following a single worthless idea on how to kill a swallow.