User blog comment:Gruntburner/Mass Effect 3 Achievements Revealed/@comment-4118072-20120204224838/@comment-4746180-20120207213905

This is the one that puzzles me the most... Of course Harbinger could be that "old adversary", but I am more inclined to say The Illusive Man or maybe, well... Don't want to spoil anything.. Let's say, something are someone you let live in the first episode that could be an enemy.