The Agiel is the torturous weapon of the Mord-Sith.
An Agiel looks to be a simple, red leather rod that hangs from a thin golden chain that is tied around the wrist of a Mord-Sith. An Agiel is in fact a weapon powered by the bond to the presiding Lord Rahl and when used deliberately by a Mord-Sith, is capable of causing extreme amounts of pain. It mortifies the skin, tissue, and muscle, and can even break bones. An Agiel is powerful enough that a particular touch can give instant death.
Agiels are used by Mord-Sith to "train"(torture) victims making them into mindless slaves. They are also used in combat, in a similar style to daggers.
Mord-Sith are created by being tortured by an Agiel, and are taught every possible form of torture with it. Once a Mord-Sith has passed her trials, she employs the same Agiel she was trained with on her victims. Once an Agiel has been used on someone to give pain, that person can never come in contact with that particular Agiel again without feeling its sting. Therefore, a Mord-Sith is in constant agony each time she uses her Agiel on her victims.
Some people have been shown to be able to withstand the torture Agiel for remarkably long periods of time within succumbing to the will of their trainers; these people are those with the strongest character, will, and emotional ties. Those with the gift are the only people who can resist becoming slaves, by magically partitioning their minds and allowing only part of themselves to be subjected to their trainer. These people are still left with a deep seated emotional scars.
When Richard Rahl became master of the Mord-Sith, he ordered all Agiels destroyed, but the Mord-Sith refused to let him as he had given them their freedom, and they chose to wield their Agiels out of their own free will. He also started the practice of wearing a deceased Mord-Sith's Agiel around one's neck as a sign of respect to her and her sufferings. Richard wears the Agiel of Raina, a Mord-Sith who freely served him and died in a plague, while his wife, Kahlan Amnell, wears the Agiel of Denna, the Mord-Sith who nearly trained him and took him as a mate.
In Legend of the Seeker, the Agiel is not a thin, simple rod but rather a baton-like stick. The Agiel could also turn white when the Mord-Sith breaks someone. It was also one of three essential ingredients necessary to travel back in time. The Agiel could be used without leaving wounds on a person, but could leave burn-like scars after being held against the skin for a long period of time because of the heat it created. The Agiel was not subject to intent, and therefore could cause pain even when its wielder intended otherwise. It is also revealed by Denna while training Richard that Agiel is more than a weapon, when one stops the heart of the victim with an Agiel a part of the torturer is placed in the victim. This makes the victim's mind merge with the will of the Mord-Sith. Thus the personality and will of those tortured is slowly snuffed out turning the victim into a mere puppet to the extent that once this process is complete the Mord-Sith won't even need their Agiel to compel one of the captives. The victim becomes an involuntarily devoted pet.