I have also come up with a new narrative mechanic for the "Heart of Darkness". When you create your warband, if you'd like, you can send them on a Quest. For example, shown below is a warband roster for "(General Name) and the Lake of Blood". A Quest is a special warband roster that includes rules that may replace some of your normal Allegiance abilities, and present challenges to your warband that show its story progression and eventually lead them to either success or failure.
If you are not playing a specific ongoing campaign with a friend, this can be a fun way to "forge the narrative" while engaging in battles with other opponents. There will be several different foreboding Quests for warbands to go on, and the best part is that once you finish one quest, you could always have your same warband sally forth on another.