User blog comment:JordanaBrewsterFAN/Mass Effect 3 4th DLC./@comment-78.8.158.162-20130119154502/@comment-24174486-20130120005659

"Harbinger was supposed to be main antagonist of the series, but it doesn't even speak a single word in ME3."

The main antagonists of the series are the Reapers as a whole. While Harbinger played a notable role in ME2 because he led the Collectors (the primary antagonists of that game), in ME3 Harbinger has very little significance over any other Reaper.

"Shepard could somehow destroy Harbinger as the oldest and the largest Reaper and leader of it's kind, they would not be so organised like ants with dead queen."

Nothing in the series implies that the other Reapers would become disorganized if Harbinger was destroyed.