Caladbolg

Caladbolg: The Rainbow Sword is the magic sword of Fergus mac Róich. It activates as a sword of a light that "stretches from the magic sword like a rainbow", allowing it to cleanly slice off the top of hills at full power. He used it as such when taking out his rage at being unable to take his revenge, slicing through the tops of three hills. It is the natural enemy of Cú Chulainn due to him having geas that makes it his duty to be defeated once by the sword so long as it is wielded by someone who is Ultser-born. It is the prototype of Excalibur Galatine, and thought to be the same sword as Caledfwlch from Welsh mythology, possibly making it the prototype of Excalibur as well. EMIYA projects Caladbolg II, modified to to improve its function as an arrow. The sword changes appearance as Saber goes through Ascension.

Fake Spiral Sword
Caladbolg II: The Fake Spiral Sword is a modified version of the original Caladbolg using Reinforcement. Archer encountered the original sword at some point during his life, adding it to the numerous Noble Phantasms stored in Unlimited Blade Works and allowing him to modify it to suit his fighting style. He projects it to be used as an arrow rather than a sword, with the improvements aimed to make use of it as an A-rank Broken Phantasm. He chants the first line of Unlimited Blade Works' incantation before firing it and, despite being renamed with the "II", he still only chants "Caladbolg" to activate it. When utilized as a close-combat weapon, he thrusts it to pierce the opponent.

The blade of the sword coils into a spiral that can penetrate any target by creating a twisting distortion in space when fired. When fired at Caster, it is powerful enough to easily break through her magical defenses simply as an aftereffect of it being purposely fired at an area far away from her. Shirou believes that, even if she had tried to teleport away from the attack, she would have simply been twisted along with the space as well. After being intercepted by Berserker, it creates a large explosion capable of razing a cemetery and engulfing it in flames.

The Fake-Fake Spiral Sword
Caladbolg II: The Fake-Fake Spiral Sword is a deteriorated version of Caladbolg II utilized by Chloe von Einzbern. Releasing its true name strengthens it and allows it to pierce everything by twisting space.

Development
Hirokazu Koyama is the weapon designer for Caladbolg II.

Gilgamesh is shown using the Caladbolg from the Gate of Babylon on Lancer in the Fate route Fate/stay night anime, but it uses the design of Caladbolg II due to being developed before the original Caladbolg appeared in Fate/Grand Order.