Darkfriends are regular humans who have sworn to serve the Dark One. Anyone can choose to become a Darkfriend, and those who serve are from all stations in life, from fishermen to queens. There are even rumors of Darkfriends among the Aes Sedai, a fabled "Black Ajah." Darkfriends are detested by the rest of society and may be killed if discovered, so they keep their support of the Dark One secret from the wider world.
Motivations for why a Darkfriend chooses to serve the Shadow vary. Some are tempted by promises of riches and power, while others seek immortality, or believe that serving the Shadow is necessary to keep the world in balance. Some are only looking to experiment with something taboo, and do not realize that vows made to the Shadow are binding in their life, and sometimes in the next.
In the Age of Legends, the Friends of the Dark formed as human followers of the Shadow. Many were promised immortality when the Dark One broke free of its prison. The Darkfriends during the previous age tended to truly believe in the cause of the Shadow much more frequently than in the Third Age.
During the Trolloc Wars, many Darkfriends fought in the name of the Shadow against the forces of the Light alongside Trollocs and Dreadlords. The armies of Manetheren stood against many Darkfriends, though it was ultimately destroyed in the conflict.
Around the beginning of the New Era, the Children of the Light formed as a military organization, with the aim of rooting out all Darkfriends from society. However, with their power growing unchecked, many extremists in the order began to consider anyone against them to be Darkfriends, and considered all channelers to be in league with the Dark One.
Padan Fain, a Darkfriend peddler, visits Emond's Field on Bel Tine as usual, drawn there by the extraordinary presence of five ta'veren in a single village, and speculating that the Dragon Reborn is among them. While concealing his true allegiances, he helps orchestrate the Shadowspawn attack of the Two Rivers through the Ways, quietly leaving when the attack begins. The people of the village believe he is dead.
Dana, the Darkfriend innkeeper of the Four Kings Inn, is sent dreams of five youths from the Two Rivers from the Shadow. When Rand al'Thor and Mat Cauthon arrive in Breen's Spring, she initially treats them fairly warmly. She later braids her hair in order to look like Egwene, and locks Rand in a room sealed with an ironwood door. After attempting to unsuccessfully seduce him, she disarms Rand and steals his father's heron-marked sword. Rand manages to break down the door, and she chases after him through town with a sword. She reveals her allegiance to the Shadow to Rand and Mat, and tells them that she wants the Shadow to prevail so that the Wheel can be broken and humanity's suffering can be ended, and boasts that she will receive the same glory as Ishamael by delivering the Dragon Reborn to the Shadow. However, she is killed when Thom Merrilin throws a knife through the back of her neck.
Padan Fain continues to follow the youths that left the Two Rivers, making it to Tar Valon, and journeys through the Ways to reach them in Fal Dara. The group realizes something is amiss, but does not know of Fain's activities, although Perrin Aybara realizes that he is alive. When the Shadowspawn pour out of Tarwin's Gap, Lord Yakota realizes that there must be Darkfriend saboteurs within the city. Fain enters Fal Dara by retrieving a password, and kills the guardswomen holding the keep. He enters the throne room and stabs Lord Yakota, Uno Nomesta, and Loial with a ruby-hilted dagger once in Mat's possession, and steals the Horn of Valere. He tells Perrin about his true goals, and although Perrin threatens to attack, he does not take the risk with many Fades still in the room.
Didn't you hear her? She's a Darkfriend, boy. Sworn body and soul to the Dark One.