User blog comment:Mexican215/So who was satisfied with the last battle against the Reapers?/@comment-2250460-20130205222107/@comment-5131329-20130206012524

If the final battle was (for whatever reason) on Tuchunka and Harbinger got attacked by Kalros?

If Harbinger pulled a Sovereign on The Illusive Man?

Those are the only 2 ways I can think of (that have occured in game) that we could fight Harbinger in the final battle. There's probably more, but still...