The Night Lords are one of the traitor legions of Chaos Space Marines.
They originated on Nostramo, a world which was trapped in a perpetual night and ruled by the Primarch Konrad Curze, who used Night Haunter as an alias while ruling the world as a merciless vigilante. he later re-adopted the title during the Horus Heresy. They ruled through terror and so were one of the most obvious of the traitor legions. Even before the Heresy, the Night Lords decorated their power armor with symbols of death, realizing that fear was a weapon effective as any bolter or chainsword.
During the Heresy Night Haunter was assassinated, supposedly by a Callidus Assassin. This misfortune, however, did not slow the Night Lords down and they continued to apply themselves to his mantra and to this day are specialists in terror. In combat, they tend to use large amounts of Raptors, Chaos marines equipped with jump-packs.
Night Lords do not tend to use Daemons in their armies, and indeed with the current rules cannot, with the exception of Furies, Daemons of Chaos Undivded who share the Night Lords fondness for brutal murder and psychological warfare. Night Lords do not worship a specific power of Chaos, instead following the concept of Chaos Undivided. Indeed, the Night Lords have a contempt for all of the Ruinous Powers, as well as weakness of any sort.