There are different types of demons that can branch out into seemingly infinite amount of other types. with their types differing with each's own objective.There are demons whose tasked to haunt bloodlines due to earlier lines of a generation having done something ... to other human beings. There are those that can spawn in later generations that will ensure another 7 generation and so on
Contrary to popular believe, almost all demons need a physical host to reside in, either an object, ring, house, rat, people, with people being the best choice for certain demons. (i say "certain" because it depends on the type of demon, and it's objective.)
People are their main go-to source for a host because they are mostly ignorant and provides energy which is their food. By instilling them with fear, doubt and lies they feed of this energy. Most of these demons will never manifest physically unless it's terrible. With that being said the ones mentioned above are miniscule in rank compared to others.
Legion is an example of such. It is single body of demons comprised out of multiple demons.
But working as a hive.