Poltergeists

Poltergeists are malevolent, invisible and intangible spirits that are native to the Milky Way galaxy.

Physiology
Notably, poltergeists do not have a physiology. They are comprised of pure magical kinetic energy.

Behaviour
Poltergeists' life span is tied to the house that it inhabits. A poltergeist cannot survive for long without human contact, or at least a plentiful amount of household items to mess around with. Thus, poltergeists can be killed by destroying their houses.

Poltergeists are incredibly hostile creatures that have an innate need to scare people and cause havoc. This is most often accomplished by making objects float and fly around, and by various abrasive sounds such as screaming and laughter.

More powerful poltergeists can also make objects disappear and reappear, deform and destroy items without moving them, and wedge items inside walls and other objects.

Poltergeists can be considered to be manifestations of restlessness. Poltergeists will calm down only if they are ignored.

Clustered poltergeists
When two or more poltergeists somehow meet, it is possible for them to merge into a more powerful being, called a clustered poltergeist. Large poltergeist clusters are incredibly dangerous, as they possess magical abilities rivaling those of Genies or even Living forests.

Clustered poltergeists can move entire houses, summon large objects out of thin air, slightly alter the movement of time, and even outright kill people.

As some clustered poltergeists are powerful enough to move their house with them, they can seek out new houses after destroying the old one. They will still die out normally if left without a house for extended periods. Houses inhabited by clustered poltergeists are thus usually burned down to stop them from spreading.

Trivia

 * Poltergeists were an invention of Satan, or possibly one of his Demons.
 * It is somewhat common for people in abusive relationships to blame poltergeists.
 * It is illegal to sell houses with poltergeists.
 * It is also illegal to infect houses with poltergeists. Humans have yet to find a viable method of doing this, but this law has been passed in many countries as a preemptive measure.