Lightslayer

A shadow in the night, the lightslayer moves with the sound of a whisper to extinguish light wherever she finds it — specifically, she extinguishes the followers of the Holy Light. The lightslayer embraces the teachings of the Forgotten Shadow and develops her personal power by slaying enemies who stand in the light. Some consider lightslayers the dark mirror of the Scarlet Crusade, but in truth the two organizations have little in common. The Scarlet Crusade possesses a firm hierarchy; their fanaticism is orderly and focused. Lightslayers, on the other hand, usually work alone. They make use of stealth, hit-and-run tactics, and the element of surprise. The only strong similarity between the two groups is the dedicated zeal with which both orders pursue their goals.

For more on this dark order, see:

http://www.wowwiki.com/Lightslayer