Fidelacchius is one of the three Holy Swords made from the three nails of Christ's crucifixion. It used to have the form of a katana, which glows in combat, sheated in a cane-like scabbard. In the hands of Waldo Butters, it has the form of a Jedi lightsaber,[Footnote 2][Footnote 3] capable of switching itself on when drawn from a coat pocket. It bears a single kanji on the hilt, which, according to Bob, reads Faith. Its weakness is to be wielded in an act of betrayal, and will be destroyed by it.
It was formerly wielded by Shiro Yoshimo, and was given to Harry Dresden for safe-keeping until the next knight could be found—through Changes. In Ghost Story, Sir Stuart Winchester informs Dresden that the blade is actually the legendary Japanese sword Kusanagi.[Footnote 1]
The first known wielder was Shiro Yoshimo, who received it from a person only known as "The Egyptian". After Shiro's death, the sword was entrusted to Harry Dresden. In Changes, it was wielded once, temporarily, by Karrin Murphy, in the fight over Dresden's child. Murphy assumed custody of Fidelacchius and Amoracchius after Dresden became the Winter Knight. It has since been taken up by Waldo Butters to be wielded in battle once more.
Kusanagi's full name, Kusanagi-no-Tsurugi,[Footnote 1] is translated as "The Grass Cutting Sword". It was originally known as Ame-no-Murakumo-no-Tsurugi or "The Sword of Gathering Clouds". The sword was said to have been forged by one of the three main deities, Susanoo-no-Mikoto,[Footnote 4] the god of storms, and given to the Emperor of Japan, the descendant of Amaterasu,[Footnote 5] the goddess of the sun and light, another one of the three main deities. According to the Imperial family the sword still exists but (along with the rest of the Emperor's treasures) is kept out of sight; however, the apocryphal nature of the sword's history raises speculation the sword is not in the Emperor's possession: its absence being concealed by the Shinto priesthood.[Footnote 1]
In the series
In Death Masks, Shiro Yoshimo fights Ursiel, the sword emitting its own silver-flame of light that steadily grew brighter along with it humming power of deep strength. When Shiro Yoshimo rescued Harry Dresden from Nicodemus Archleone, he handed it to Dresden to take out with him. After Shiro's death the Sword is left under Dresden's guardianship.
In Proven Guilty, Michael Carpenter asked Harry Dresden if he found a potential new wielder of Fidelacchius. He suggested to Dresden that he start researching its history. It's not the first time a Swords of the Cross has been held in custody by a member of the White Council. The Original Merlin once had custody of Amoracchius.
In Small Favor, Harry Dresden made a deal with Nicodemus Archleone: Fidelacchius for the Archive. Dresden discussed the deal with Michael Carpenter. Dresden considered the power of the sword and what use he may or may not put to it, what would unmake it—treachery would. (Plus it's description.) While they faced off Nico and the Denarians on Demonreach and holding up the Sword, Dresden suggested that maybe they were facing three Knights instead of two. Dresden tossed the Sword into the air trusting Michael would catch it. Michael fought the Denarians with Amoracchius in his right hand and Fidelacchius in his left, killing a Denarian beast with a slash from each sword.
Michael dropped Fidelacchius when he was shot while being airlifted off the island. Dresden retrieved the Sword. Trying to escape the island on a boat, Dresden was ambushed by Nicodemus parting a shadow. Nico thought Lasciel disabled Dresden, reached for Fidelacchius, then Dresden strangled him with his noose. As Thomas was getting Dresden on the Water Beetle, Deirdre Archleone attacked and Dresden turned so that Karrin Murphy could draw Fidelacchius from his back. The Sword glowed bright as she drew it only an inch and Deirdre vanished. Karrin asked what that meant and Dresden said that he thought that was a job offer. Karrin answered that she has a job.
In Changes, Harry Dresden buried Fidelacchius and Amoracchius with Bob in the Leanansidhe's garden defenses in the Nevernever. Lea returned the Swords and Dresden entrusted Fidelacchius to Murphy and Amoracchius to Susan Rodriguez for the mission to rescue Margaret Angelica from the Red Court. In Chichén Itzá, when she wields Fidelacchius, an Angel speaks through her and she cuts through the Red King's spell breaking it. Murphy charged through hundreds of Jaguar Warriors, the sword glowing, and kills one of the Lords of Outer Night.
In Ghost Story, at a Chicago Alliance meeting, Daniel Carpenter let slip that Murphy has two of the Swords of the Cross there in her possession there in her home in front of Felicia Raith, a White Court vampire, endangering the Swords.
In Skin Game, when Nicodemus Archleone confronts Harry Dresden and demands that he kill Butters at the Carpenter's house, Karrin Murphy threatens him with Fidelacchius. Originally nervous, Nicodemus realizes that Murphy isn't a true Knight and offers his surrender, relinquishing the noose from around his neck and dropping his Coin on the ground, before ordering the Genoskwa to kill Dresden. Enraged, Murphy swings the sword in an attempt to kill Nicodemus. As he had already surrendered, using the sword was an act of betrayal, and Nicodemus was able to wrest the sword from her and destroy it against the pavement, leaving only a broken hilt behind. Later, Dresden throws it to Charity Carpenter in an act of desperation, as they were about to be killed by Nicodemus and Polonius Lartessa. She missed grabbing the hilt, which bounced and landed in the hands of Waldo Butters. As he took it, the hilt drew upon the power of faith to become a lightsaber which severed the blade of Nicodemus's sword. Faced with the renewed Sword of Faith, Nicodemus fled in terror.
In Peace Talks, Waldo Butters spars with Sanya using the lightsaber-form Fidelacchius against Esperacchius. Waldo slashes through Sanya's wrist but causes him no harm. It does not damage Esperacchius, either. The two Knights, Michael Carpenter, and Harry Dresden realize the Sword can't harm mortals (at most it feels uncomfortably warm to Dresden), but its ability to attack supernatural foes has increased, as the sword can be no more or less powerful than it was created to be. In response, Sanya suggests that Butters carry a gun to deal with mortal opponents. It is also suggested that the Swords are inhabited by Angels, in balance to the coins being inhabited by the Fallen, and that this sentience allowed the sword to see into Butters mind and adopt the appearance of a lightsabre in order to continue serving its purpose. 
- Small Favor, ch. 06
- Skin Game, ch. 51
- Death Masks, ch. 33
- Changes, ch. 9
- Changes, ch. 46
- Changes, ch. 39
- Ghost Story, ch. 9
- Cold Days, ch. 27
- Skin Game, ch. 50
- Death Masks, ch. 22
- Proven Guilty, ch. 47
- Small Favor, ch. 39
- Small Favor, ch. 40
- Small Favor, ch. 42
- Small Favor, ch. 43
- Small Favor, ch. 44
- Small Favor, ch. 45
- Changes, ch. 12
- Changes, ch. 38–39
- Changes, ch. 47
- Skin Game, ch. 29
- Peace Talks, ch. 18