The Nightstalkers are more than mere ghosts: they are the stories by which parents scare their children abed. They are the monsters that dwell in the darkest forests, who demand sacrifice from superstitious villagers. They are the darkest dreams and ill omens made manifest.
Stalkers take many forms – often perceived by different races in different ways depending on the superstitions and fears of a culture or individual, or a hideous perversion of their once-mortal form. As in the life they were so cruelly ripped away from, there are many different types of Nightstalker, sometimes even shaped by remnants of memories and personalities of their lost mortality. Sometimes, when the powers of the Abyss are strong, the presence of the Nightstalkers rips a portal in the fabric of reality – a doorway between dimensions.
– lore snippet from 3rd Ed. Rulebook