Ghosts

Ghosts or Impressions are a kind of spiritual entities, type IIa spirits, created largely by a single source of sentience. As a consequence, Ghosts often bear strong resemblance to the source that created them in their "personality", attitudes and behaviour.

Ghosts themselves are not sentient spirits (not sources of sentience) but just accumulations of an individual's thoughts and emotions.

Appearance
Ghosts often take on an appearance similar to the source they were created from, but usually distort or change it. They are not physical entities and don't interact directly with the physical world, but are able to create auditory signals and visual effects seemingly made out of fog, dust or lights.

Formation and "life cycle"
Ghosts form naturally around sources of sentience. However, only the vast minority of sources form a ghost. Usually, the formation is triggered by intense emotions - love, fear, hatred, pride - that form the core of the spirit.

Contrary to what the name might imply, ghosts can form around living individuals and do not necessitate a death. Similarly, they are independent entities, unrelated to the person that formed them, and are not parts of a "soul". Although revered by some, ghosts are nothing more than echoes of powerful sources of sentience.

Like other spiritual entities, ghosts acquire more and more sentience over time after they have formed. Unlike some other spirits, ghosts dissipate when they acquire more unrelated sentience than what they have at their core. When this happens, some sentience might eventually coalesce as a more nebulous genius loci kind of spirit.