They seek to:
Kill Whitelighters; coerce future Whitelighters into taking their own lives; spread evil through procreation; assassinate Whitelighters who are about to fall from grace and those yet to "earn their wings" so that they die without fulfilling their destiny or redeeming themselves.
Darklighters are fictional characters in the WB television series Charmed. Darklighters are the demonic counterparts to Whitelighters, fictional guardian angels in the series. They are sometimes referred to as Blacklighters.
Darklighters can teleport from place to place by orbing, as Whitelighters can, but their orb effects are black in color. Their ability is called black orbing. They also come equipped with crossbows, which they can summon into their hands at any time. The Darklighter crossbow arrows are coated with a special type of poison that is especially lethal to Whitelighters. Some Darklighters also possess the touch of death, a power triggered by hate and focused through their hands that allows the user to burn someone to death – in contrast to a Whitelighter's ability to heal others. Similar to Whitelighters, Darklighters also possess the ability to sense another magical being's presence through a magical form of telepathy. Hence they can locate a Whitelighter anywhere on the globe by merely concentrating.
The primary goal of all Darklighters is to kill Whitelighters in order to make witches more vulnerable to an evil attack. In addition, Darklighters often work as hired guns or mercenaries to higher level Demons and Powers. They generally work with Warlocks or demons to accomplish their goals. Another goal of darklighters is to change or kill future whitelighters so they are unable to perform the 'good' they are meant to do later on in their lives.
One type of Darklighter is an Assassin, who strives to kill current Whitelighters to leave their charges vulnerable or to kill future Whitelighters before they can earn their wings.
Another type of Darklighter spreads evil through procreation, by seducing innocent mortal women and tricking them into bearing their progeny. It's against the rules to tell these women about who they truly are or fall in love with them.
A third type of Darklighter mentioned on the show is known as the "Spirit Killer", who drives paragons of good to suicide. They curse them with bad luck, following the victim around, invisible to everyone, including the victim. As the bad luck escalates and drives all thoughts of happiness from the victim's mind, the Darklighter is there every step of the way, whispering hopeless thoughts into the victim's ears. Once the victim is thoroughly depressed beyond recovery, the Darklighter changes his whispered words to encourage them to commit suicide. Whitelighters-to-be are particularly favorite targets of Spirit Killers; if the person commits suicide, it automatically disqualifies him/her from becoming a Whitelighter.
A fourth type of Darklighter is called a Tracker. A Tracker is a Darklighter who goes after Whitelighters who are about to lose their wings. Their orbing is more of a spiral than the particle of other Darklighters, and they are considered to be much more powerful than a regular Darklighter. They are rogue Darklighters who often only answer to themselves. They distinguish themselves from other types of Darklighters by wearing tribal tattoos on their face. Only one Tracker Darklighter, Ronan, has been shown, who the then god-like Cole imbued with additional powers in order to destroy the Charmed One Paige.
Darklighters are not invulnerable and are thus open to many ways of being vanquished. Previous methods used were potions, athames, energy balls and witch powers such as molecular combustion. Darklighters also appear to be susceptible to their own Darklighter arrows. However, some Darklighters are shown to be able to survive one of their own arrows.
Alec – The first Darklighter seen on the show. Alec fell in love with a mortal woman, who he was assigned to impregnate. He also possessed the touch of death. He was killed by Prue who switched powers with him and killed him with his own touch of death.
Spirit Killer – The second darklighter seen on the show. He tried to make Maggie Murphy kill herself by cursing her with bad luck and giving telepathic voices, but he was stopped by Prue and then by Leo. He tried to force Prue to kill herself but was stopped when Leo was given his powers back and he orbed him to parts unknown.
Damien – Damien was a Darklighter who was sent by Gideon to kill Leo Wyatt. He was vanquished by his leader with a poison arrow in order to stop him from revealing that Gideon sent him.
Ronan – A Tracker Darklighter blessed by Cole with additional powers. He was used by Cole in an attempt to get the Charmed Ones to vanquish him then vanquished by Cole with an energy ball.
In the finale of Season 6, it is revealed that there is an evil parallel world that exists in tandem with our world. In this world, Leo Wyatt has an evil counterpart who is a Darklighter rather than a Whitelighter. Paige Matthews, Chris Halliwell and Wyatt Halliwell also have evil counterparts in the same alternate universe who are half-Darklighters as opposed to half-Whitelighters.
Salek – Upper level member shown to have trained new Darklighters. He was blown up by Piper after mortally wounding Paige and killing her charge. Paige's charge became a Whitelighter anyway and saved Paige with her new healing power.