This fighter subclass was inspired by a number of pop culture items from books to games to shows.
Possessed fighters have a second spirit inside of them. This spirit is either a restless spirit form the afterlife or a disimbodied fiend from the dark corners of the Abyss or nine hells. You can have whatever relationship with the spirit that you want: they could be a friend or mentour, they could be a rival or antagonist, or they could be a love intrest (looking at you Dresden Files). The increase in power you get from leveling up can be flavoured as the spirit taking more control, as the two of you coming to an agreement, or you beating it into spiritual submission and using its power on your own terms.
Mechanically you choose either spirit or fiend at 3rd level. This choice affects most of you other abilities and cannot be changed. The abilities unaffected by the choice allow you to frieghten other creatures by taking on your spirits visage and the ability to harness their knowledge for your own.
- from the publisher's blurb