That's a good question and in part that is one reason I wanted to search the scriptures to find out just what is written about demons and the power of darkness. As I grouped two categories of Possessed and Captivity together. There's seems to indicate that at least captivity indicates the fight against being overtaken by the power of darkness.
Now what seems to me is that alot of these terms we used to describe like 'darkness' has a deeper meaning. I mean in the sence of it's symbolize something else. Here 'darkness' in scripture indicates the same as being 'blind' or being 'asleep' in the 'night' as Paul alludes to in 1 Thess.5:4
This has little to nothing to do with being demon possessed or does it? This captivity is seem to be the ones that are hold in the darkness of disbelief of which Jesus said he came to free.
In the case of Mark 8:28-34/ Luke 8:26-40 'Demons are casted into swine' there seems to me to be a greater meaning rather than reading this as a literal event, it rather seems to speak of a parable of sort.
The unclean spirits (demons) plead with Christ that he cast them not into the sea. Again 'sea' is a symbol to denote 'peoples'. Could there be a relationship in that the unclean spirits are a symbol of those that speak evil which disbelieve to those children of Ephriam that are driven into the sea of the Gentiles?
As to some would declare that Jesus when speaking in Luke 16 'Rich man and Lazarus' is a literal event, but to carries many of the same symbol's of a parable. Which relates to the Rich man as Judah and Lazarus as Eliezer to denote 'not my people' will be called my people the children of Ephriam. So you see there can be an alternative view of demons.