Immortal Stone Knight Blueprint
I've had this happen on 2 playthroughs, this is the first I've encountered on this game and the last game; I had atleast 2 like this. The stone knights BLUEPRINTS are immortal, I have to recall my minions; otherwise they will fight them till they kill themselves basically. I place walls around them so they don't lure all passing minions to this endless fighting trap.
I've had no luck finding any save files.