Knights of the Cross

The Knights of the Cross, also known as the Knights of the Sword, are a group of people who wield one of three holy swords, each forged with one of the nails from the Crucifixion of Christ.

Description
The knights are a force for Light that help keep the balance with the forces for darkness. When something truly evil happened, one of the Knights tended to show up.

The exact prerequisite qualifications for being chosen are unknown. Harry Dresden's working theory is that Knights are descendants of old royalty - which were the subjects of power in the old world. This would fit all the recent Knights known to him: Shiro is descended from the last king of Okinawa, Sanya from Saladin, Michael Carpenter from Charlemagne. So far, the swords have been temporarily used by at least three people of unknown descent, Karrin Murphy, Waldo Butters, and Susan Rodriguez.

The main reason for the Knights to exist is to fight the Order of the Blackened Denarius. Whenever they are up to something, one or more of the knights tends to show up to combat them. Michael later explains that their main enemies are the Fallen, and that their job is to save the humans who hold the coins and help them free themselves of the Fallen.

Between Small Favor and Cold Days, Sanya is the only active Knight left. At the end of Skin Game, Waldo Butters joins him.

Dead Beat
In Dead Beat,

Proven Guilty
In Proven Guilty, the Knights of the Cross were founded to save the Denarians, not to destroy them. Their purpose is to face and challenge the Fallen Angels and give the human they hold the chance to reconsider their choice, to set it aside, and to change their path.

Small Favor
In Small Favor, Michael Carpenter states that their purpose, at least as it relates to the Denarians, is to rescue the poor souls trapped by the Fallen. They were not given their power to strike down their enemies no matter how much they might deserve it. When Harry Dresden attempts to persuade The Knights into taking an offensive operation against The Fallen, Michael Carpenter answers, "This isn't a democracy, Dresden. We serve a King."

Death Masks
In Death Masks, Father Forthill tells Harry Dresden that Nicodemus Archleone is an ancient foe of the Knights. He says that Nicodemus has killed every knight who has gone up against him, all except two. Shiro Yoshimo is one of them. Dresden thought it was the Knights job to deal out justice: Fists of God.

Changes
In Changes,

Cold Days
In Cold Days, Karrin Murphy tells Harry Dresden that she is not telling him where the Swords—Amoracchius and Fidelacchius—are and that he’s not getting them back. She knows how powerful they are and how vulnerable they are in wrong hands.

Skin Game
In Skin Game, Karrin Murphy, Michael Carpenter, and Waldo Butters all wield a Sword at one point. Murphy temporarily uses Fidelacchius to thwart Nicodemus Archleone, but plays into his plan and results in the destruction of the blade. Michael picks up Amoracchius again to help Harry Dresden. Butters takes possession of Fidelacchius hilt to fight off Nicodemus Archleone, and the Sword's broken blade is replaced by a beam of energy. Waldo Butters accepts to wield Fidelacchius long-term. Dresden decides to hide Amoracchius beneath Demonreach.

According to Nicodemus Archleone, the average Knight of the Cross served for three days; most wielded a Sword for a specific purpose and was relieved of their duties after fulfilling it (such as Karrin Murphy, who fought in the battle of Chichén Itzá). If this is true, then long-serving Knights like Michael Carpenter and Sanya are more of an exception than a norm.

"Day One"
In "Day One", Waldo Butters takes up is first mission, fighting a baku causing sickness in people by feeding them nightmares.

Word of Jim
On separate occasions, the author has revealed that the people who got to temporarily wield a Sword of the Cross include famous sultan Saladin in the Crusades and the first American President George Washington with Esperacchius in the Seven Years' War.

External references
Charlemagne
 * Charlemagne Video - History.com
 * Charlemagne - Wikipedia
 * Charlemagne -- Britannica Online Encyclopedia
 * The Sword of Charlemagne -- myArmoury.com

Saladin
 * Saladin — History.com Video
 * Saladin - Wikipedia
 * Saladin (Ayyubid sultan) -- Britannica Online Encyclopedia
 * Saladin - Timeline
 * Saladin (Salah al-Din Yusuf Ibn Ayyub)

Last King of Okinawa
 * Ryukyu Kingdom - Wikipedia
 * Shō Tai - Wikipedia
 * A Brief History of Okinawa
 * List of monarchs of Ryukyu Islands - Wikipedia