When a person dies by being betrayed or missled, their sould screams out for vengance, the sound ringing through out the halls of Mount Olympus. Most of the gods ignore the sound, brushing it off as one would to a fly. But not the firey haired goddess of the moon and the Hunt. No, Artemis hears it as an apportunity to have someone else serve her. She then offers them the choice to become Dark-Hunters in return for a single Act of Vengeance. Once the bargain is accepted, the new Dark-Hunter is branded with Artemis's double bow-and-arrow mark.
In exchange for their souls, Artemis allows all new Dark-Hunters twenty-four hours to exact vengeance on those who wronged them during their human life. Most notably the Dark-Hunters usually go after the person(s) who killed them. After the twenty-four-hour period is up, they belong to her, and Acheron, leader of the Dark-Hunters, gets sent to train the new dark-hunter(s) in their new life and how to use their powers. After the training with Acheron is over, the Dark-Hunters are assigned a location somewhere in the world and spends the rest of eternity fighting with Daimons and other evil.
Artemis pays them well for their services and provides them with human helpers known as Squires.