I finally got around to unlocking Dragoon, and just completed the level 35 quest, where Ser Alberic told me the story about Ishgard's founding and the skirmish with Nidhogg. (i.e. the plot point for Heavenward mentioned at Las Vegas fan festival)

One thing I can't help but notice in the tale is that Thordan seemed to start it by attacking the wyrmking unprovoked. Obviously Ishgard is going to be extremely biased, but the tale as told doesn't exactly indicate Nidhogg approached with hostile intent.

Though given Ishgard's shady nature and who they may or may not be harboring within their walls, I do wonder if the missing Eye and ensuing blood feud is not the only "sins" the rising chorus is preaching about.