Demon has, unfortunately, become a pretty broad term, often used to refer to anything scary and supernatural.
A more accurate description (as used by Wyrd) is that demon is any entity that is a hybrid of a living host and one or more spirits. Not to be confused with possession, where a spirit takes over a host, but an entity that is a true combination of both and could not survive separation. A demon is not a dual entity, but a true merger of the two parts to create a new whole.
Given the variable nature of both living beings and spirits, there is no definitive list on what traits, appearance, or weaknesses a demon may possess.
Asmodeus, Bhaal, Hungry Ghost, Rakshasa (ascendant race),