This is a part of the "For the King" campaign, and for the sake of that being the aptly named scenario for this game, is marked as main storyline.
Upon starting the campaign, the Act I questline will take you into a cultist camp to deal with the Chaos Leader enemy. While you may find these type of enemies in later scenarios, I do believe that this is counted as part of the FTK standard campaign questline. Just be careful, as these enemies will use chaos-empowered attacks that can inflict random statuses on their targets if successful.