Just a State of Mind is a (mostly) unpublished work of fanfiction for Call of Duty: Black Ops III. 10 chapters of the work have been released, but a large majority of the story remains to be published.
The story begins immediately after the level "Demon Within" from the game, and follows Paszek, the game's protagonist (who has now been given a proper name) throughout the remainder of the game's campaign and far, far beyond it.
Although something of a fan novelization, Just a State of Mind greatly focuses on the internal struggles of its characters, more so than the action itself, and introduces some notable original characters.
As of yet, it is the only fanfiction, outside of one-shots, to expand on the setting of Call of Duty: Black Ops III.