Apparition

Seen only by those with Witchsight, Apparitions, also known as Harbingers, are as real as Daemons and as real as the Dark Gods themselves. Their existence is a window into the very nature of Chaos. Without the existence of Mankind, Chaos would have little power, since Chaos and its minions are reflections of the psyches and nightmares of mortals. Feeding on fear and misgiving, Chaos takes shape, becoming the very thing mortals loathe. Apparitions are formed from these same emotions, but are defined by the common myths and legends that have circulated amongst spellcasters since the Elves first dabbled with the Winds of Magic. They are what the Magister expects to see and nothing else. With each legend of the Dark Hounds or sightings of the All-Knowing Serpents, Magisters unconsciously look for these things, casting out their witchsight to see if they are being watched by their enemies, by the Daemons who lurk in the shadow of every spell they cast. Hence, they are both real and unreal; they are the manifestations of expectations, of the pervasive terror of what lies beyond the veil of reality.

List of Known Harbingers

 * All-Knowing Serpents
 * Dark Hounds
 * Eyes of Nurgle
 * Fat Man, the
 * Handmaidens, the
 * Lost Child
 * Malbrothax
 * Riders in Red
 * Rotwyrms
 * Whispering Darkness, the

Source

 * Warhammer Fantasy RPG 2nd ED -- Realms of Sorcery (pg. 80).
 * Warhammer Fantasy RPG 2nd ED -- Tome of Corruption (pg. 208-209).