Who is Denna the Mord-Sith from Wizard's First Rule and what is their importance?

Mord-Siths are the most feared beings in the three lands, for good reason. They are able to turn anyone's magic against the victim. They specialize in training victims with horrible torture. A Mord-Sith will mate with their best trainees, inflicting even more pain for the few weeks until the victim dies. They wear red leather that is frequently splashed with the blood of their victims. The Mord-Sith Denna captures Richard and breaks him so that he will answer any question Darken Rahl asks. She takes Richard for her mate.

Mord-Siths are chosen from the most sensitive, empathetic young girls. Like all Mord-Siths, Denna had been trained from the age of 12 to 15 by a Mord-Sith. At 15 she had been mated to the Mord-Sith and finally managed to kill the Mord-Sith at the age of 18. She was then tortured continuously for two years in punishment. Like all Mord-Siths, after Denna was broken, she had to watch her mother being tortured and broken, and then do the same to her father.