Nightstalker

Nightstalkers have haunted the dreams and stories of children since the dawn of mankind and probably even before that. Like some other demonic creatures, they seem to feed, at least partially, on the fear of their victims. Divination magic revealed that they are no native denizens of our realm, however, the question where Nightstalkers actually come from is still unanswered. They are cunning and intelligent creatures that are even able to talk our language. They prefer lairs in dark and abandoned places like ruins and caves. Sometimes, Nightstalkers form small colonies, but most of them are notorious loners.
Sages speculate that the Nightstalkers' wish for loneliness results from their life cycle. At certain stages in their life, they leave the company of their kin to become lonely prowlers that secretly stalk the settlements of civilised creatures to haunt particularly their children. During these stages, Nightstalkers seem to dematerialise, almost like ghosts. They feed on the fears of those around them and adopt features derived from these subconscious fears. Luckily, Nightstalkers also gain a certain vulnerability in this state. After some while, Nightstalkers gradually retransform to their original state. Once they have reached it, they seem to hibernate for a while, which surely explains why they tend to leave civilised places in advance of the transformation. Thereafter, they live for a longer period in their natural state in which they might be encountered at remote places. During this phase, they do not actively prey on creatures but are more than ready to rip anyone in pieces who gets close to their lairs.