Spectres are malevolent creatures that feed on Dust. Spectres are created from the void between worlds. Most commonly, a spectre is thus created for each new portal that is cut open with the Subtle Knife. When Lord Asriel cuts between many worlds by harnessing the energy given off from the intercision of Roger, the void that was created released a fog storm over Cittàgazze, and produced the bulk of the Spectres in the city.
They appear in the second and third volumes, The Subtle Knife and The Amber Spyglass, though the Northern legend of the Windsuckers in the first volume, Northern Lights, tells of 'a kind of shimmer in the air' which has similar strength-sapping capabilities to the Spectres on contact; presumably anyone who had had more than a slight touch would not have survived to tell the tale. The Windsucker legend claims that they can be snagged on brambles, which would appear to be at odds with all known Spectre attacks, though any precise details should be thrown into doubt by its status as a legend and not a scientific account.
These beings are invisible to pre-adolescents, and cannot harm them. To adults, they appear as slight shimmering in the air, and move with malevolence. If they catch an adult, they eat the adult's soul out, leaving the person as a lifeless zombie-like entity. If they envelop an external dæmon, they appear to perform the same process but much more slowly. When travelling, all human groups in Cittàgazze are required by law to contain a man and woman on horseback to flee and look after the young in the case of a spectre attack. Spectres did originally not have the ability to fly, so air travel over Cittàgazze was the only safe means possible for an adult to cross the city. Toward the end of The Subtle Knife, Mrs. Coulter teaches the Spectres how to fly (although they are never seen to use this ability later).
Spectres cannot be killed by humans (or any physical entities at all; any weapon used on it will simply pass through it), but are warded off by the Subtle Knife. As such, Cittàgazze, a city infested with them, is bereft of all adults. It has been speculated that spectres, or similar creatures, may exist in our universe, but those who sense them are viewed as being mentally ill - the character Will wonders if this might affect his mother. However, this is unlikely as Cittàgazze seems to operate in the same way as our world but every adult can see them there; there are also no incidences in our world where spectres attack people.
Angels have some means of neutralizing Spectres and ghosts may be able to combat them as well. Humans whose dæmons have been removed from them via intercision can pass them without being attacked. Stanislaus Grumman uses his skills as a shaman to control one and send it onto a zeppelin to attack the pilot, causing the zeppelin to crash. Mrs. Coulter tells Spectres that following her command would give them greater prey and was thus able to control them, also teaching them to forget that they are earthbound so that they may fly.