Monotheistic religions, such as Christianity, Judaism or Islam, tend to see demons as disturbingly malevolent beings, causing things to break or fail, or moving animals towards violence or friends and family towards evil. Polytheistic religions, on the other hand, such as Hinduism or animism, tend to view demons as potentially friendly, so long as they are placated and respectful. It is arguable that the inflexibility of monotheism that causes demons to grow jealous and vindictive, as a monotheist will refuse to acknowledge the demon in a positive way ~ and that the demon within an object is merely responding to the way it is treated. Monotheists do not tend to look at demons this way, seeing them instead as unwanted residents in objects, animals or creatures that ought to be exorcised out, found and destroyed.
Demons, like the gods, are made manifest from thought, the wakening of life from the invention of intelligent beings on the Prime Material, which gives power to the gods and creates the outer planes, or unknown universe. This means that demons are not, as some suppose, older than civilization. Rather, the conception of early intelligent creatures, anxious to explain the habits of things, supposed that these things would be invested with spirits, which in turn created the very spirits that transformed into more and more complex demons. Having been made in part from curiousity and in part from nightmares, demons grew in purpose until they became the caretakers of the dead, particularly within the underworld regions of Tartarus and the Abyss.
The vast majority of demons that exist anywhere are relatively benign spirits, without form or even consciousness, fulfilling their roles as operators of inventions, tools, non-intelligent creatures as trivial as bugs or flies, acting in concert with more intelligent beings and acting in a largely random, sometimes helpful and sometimes hindering manner. On the other hand, more complex demons, such as manes, hezrou or vrocks, are self-aware creatures with higher motivations, acting as combatants, messengers or servants within the plans of Gods and Demi-Gods.
Because of their ability to possess a host, demons are far more likely to be sent into the Prime Material plane to carry out a master's wishes. When doing this, demons will not wish to appear as what they are. As they cannot shapechange, they must first seek out a host to possess, then go about their business in the guise of that host. Most demons that are employed in this manner will serve gods known for their cruelty and hatred, and so will be malicious themselves ~ but this does not necessarily describe the behaviour of all demons.
Occasionally a demon, separated from its master and completing its task, will make an effort to remain on the Prime Material. Such a demon may move from host to host or may select a host they will retain for years, while the body ages, until the host dies. Many such demons will be among those who are ready to act decently or kindly upon occasion, this being part of the reason they do not wish to return to the lower planes ~ because the caretaking of the dead does not suit them. A few demons have, in fact, proven to be great benefactors, living as powerful kings, religious leaders, philanthropists and artists. Some have served the very monotheistic establishments that condemned them, under the noses of Popes, Emperors and the wisest of the wise. No one can ever know for sure which of those figures of the past were truly self-directed mortal beings or the host shells of immortal demons.
Demons are immortal with regards to the Prime Material Plane. They can only be killed if destroyed while dwelling upon the Outer Planes. Some lesser demons, conceived early in history, are more than 40,000 years old ~ but these are the less intelligent, simpler beings of their kind. Most of the more powerful demons, those that are truly terrifying in their abilities and power, are no more than a few hundred years old.
Acedia is a state of listlessness and torpor, recognized in those who live a monastic life. It is recognized that those who suffer from the mental disease are especially susceptible to demonic possession.