The following essay comes from very deep spaces of mysticism and practice, experience, and understanding. This is only for practitioners of ceremonial magick of all traditions spanning the east and west. If your background is Occidental and your practice is of the Hermetic, O.T.O.. Chaos, etc. ceremonial magiqal tradition you already understand the complexity of the entangled hierarchies of realms, from the chthonic nadir to celestial zenith, and if your background is, say, Tantrayana, Vajrayana, Shugendo, and so on, you also will have the same understanding, with the added realms of enlightened beings outside the pale of transmigration. There is a full Mandalic scope in question here.
Unfortunately the Christian-Islamic trajectory is Zoroastrian-Aristotlean in its either-or dualism; it is an artificial view of reality. The paradigms of necessity force everything into procrustean grids, one labeled 'good' and the other labeled 'evil,' with the former being anything pertaining to their paradigm, and the latter being anything to do with what is outside their paradigm. At the most basic level of being there is duality, but the poles are always interlocked, interfused, and interdependent; there is never a mutual exclusion of anything to the point of something becoming utterly independent of anything else.Take the human species. There are many races of two basic sexes. Same thing with the animal world in general of mammals, reptiles, amphibians, etc. There is no labeling of good or evil possible with respect to factual existences in general. They just are, and everything is mutually interdependent in one highly refined ecosystem. Then we enter the domain of supernal entities of all kinds, from celestials, gods, titans, sprites, ghosts, and demons. And each of these realms has its own unique "taxonomy" of entangled hierarchies of beings. So now that I've got the "disclaimer" above out of the way (which shows, I hope, that I don't treat such subject matters in any kind of "heavy metal," jejune stupor of being a rebellious provocateur; this is a subject matter that will enhance anyone's world view), let me get down to this concise demonology. And concise it is. Beings of the chthonic realms have been given an inaccurate rap by paradigms of dualism, and by Hollywood. They are made out to be all powerful, able to take possession of bodies, and so on. Well, if a possession were to take place, one being could not do it. The biological domain is far too "dense" and gross for a singular being to take over. It would require multitudes of them, if they were to band together into a unified field, to possess, say, an animal or a human being. This aspect is actually evinced in the christian exorcism account in the synoptic gospel of the possessor(s) saying "I am legion." Since it takes an entire nether-field of demons to possess a person, a demon taken in and of itself is perhaps like a bacterium. It participates in a larger field, but individuated, its influence on its environment is negligible to nothing. Now there is another aspect to the chthonic demon field, and it is that of protection. Believe it or not, if you are the "correct" type of practitioner, they will become your protectors. Don't go to that place of thinking "evil." No, not at all. The chthonic realm is of beings of the highest level of suffering, low self esteem, melancholy, rage, and torpor. Some of them, due to their karmas, cannot break free in any way from their Sisyphus-like imprisonment and infernal torture (which ultimately was brought upon themselves). Everything is changing, and poles are shifting (literally and symbolically). Nothing is how it appears, not any more. As an aside, speaking of possessions, it is far, by far scarier to be possessed by a god or titan. This is because they do have, individually, mighty power, and they have very strong egos. If a god or titan is not your protector, then beware if they try to get your attention, and claim you for themselves. In the world of practice, you shouldn't stick your neck out, dabble, or hedge your bets if you aren't willing to pay the price. In many practices, the price is too high. Better just to wake up, because the world will then be your oyster.