A Demon is a hostile immortal supernatural creature that seeks to cause pain and misery to the mortal world, but in order for a Demon to do so, it must have a portal (a bridge) between the spirit world and the mortal world, and it must be open via occult ritual.
In order for Demons to cause pain on earth, and spread evil, they must possess a human soul (or body). Demons can be returned back to the spirit world via exorcism, but they are difficult to exorcise.
Demons have the ability to shape-shift, attach themselves to objects, haunt, and possess. Demons shape-shift to prey on the fears or emotions of the people (or person) it haunts, and they are people on Earth who follow demonic occults, Satanism is sometimes one of these occults, but Witchcraft religions and cults, and other Occults are design to cast spells and perform rituals to open gate-ways for hostile spirits to spread evil on Earth.
Religions have shown mythologies, stories, and others of Demons, although ignored and rejected by Science, people who are religious, supernaturalists, and occultists have fought against each other to preserve the good of Earth, and spread evil on Earth.