Spirit

'Spirit' is a general term referring to supernatural ephemeral beings who have a connection to human emotions. They have an affinity to a particular emotion, which determines their powers, motivations and weaknesses. They desire the spreading of their emotion, as well as the forming of bonds with humans, enhancing their ability to interfere in the mortal world.

Background
The motivations of spirits appear alien to humans. They have but the desire to spread a particular emotion, but it is unclear as to whether depriving them of it weakens or even kills, or merely frustrates them. Hotspots and other locations of mystic power appear to enhance their power and ability to manipulate the perceptions and emotions of humans.

Through forming bonds with humans they form a telepathic link, and seem to possess the human at all times, though struggle to wrest control completely away from it's original inhabitant. Outside of this, their ability to influence their human is greatly increased, able to conjure feelings, ideas and memories.

It it possible to form wards against the influence of spirits or even banish a spirit entirely. Either way, researching the spirit will be required, to find the particular weakness of that type of spirit. The presence of the substance causes immense pain and damage to both the human and the spirit. The ritual takes the form of a mock fight, duel or contest, with one person representing the material, and another spiritual plane, in which the representative of the material must be seen to triumph over their adversary, followed by striking the possessed human, finishing the exorcism

Known Facts

 * Have the power to manipulate the minds of humans
 * Have no physical form, though are visible to some psychics
 * May make deals with mortals, allowing them to possess the victim
 * They have specific powers and weaknesses based on their type - Anger spirits hate chalk

Theories

 * Spirits are leftovers of dead humans
 * Spirits are reflections of the collective human psyche
 * Spirits are alien beings attracted to human emotions
 * Spirits do not understand many human concepts such as lying, dreams or sarcasm
 * Spirits may prefer people who have more innate magical power, seeing them as "greater"