Crystal Skulls are a type of artifact which strongly resemble crystallized skulls of one of the humanoid races. Most of the skulls discovered so far seem to resemble the skulls of humans (though their proportions are never quite identical), although skulls resembling those of most other races are known. Additionally, some skulls have a curiously elongated form which resembles no known race. These invariably provoke a profound sense of unease among observers.
It is known that crystal skulls are intelligent, and are capable of affecting their surroundings telekinetically, although they never do this while they are being observed. When someone returns to a room where a crystal skull is kept, various items in the same room - including the crystal skull itself - might be found in different locations. Attempts to prevent this effect by keeping the skulls under constant observation have always ended in failure - somehow, the guards always end up asleep or otherwise distracted. It is suspected that the skulls have some sort of telepathic powers as well which causes this, although they have never directly addressed someone telepathically. However, some people who have been in contact with the skulls have reported disturbing dreams as a result of that contact.
It is known that the crystal skulls have distinct personalities - one skull, for example, vandalized any item colored in green which was put into the same room. Another has created coded messages with a set of movable letters it was supplied with, while a third one has become fond of playing chess with its owner. If several skulls are placed into the same room, their powers of manipulation seem to be magnified when they cooperate with each other - although there have been cases where certain skulls have displayed a profound dislike for each other, with disastrous consequences for anyone nearby.
Crystal Skulls are hotly sought after because their owners tend to accumulate vast influence and significant fortunes in short order - possibly thanks to the psychic manipulations of the skulls. However, several people who owned crystal skulls also came to bad ends when their skulls vanished. Whether this was because the skulls were stolen or because the skulls themselves wished to disappear is uncertain.