system
Given the following post, respond as if you're commenting on Reddit:
human
[40k] Is the Horus Heresy a secret?

So I know that part of the duties of the Inquisition is to suppress the existence of chaos from the citizens of the Imperium. Does this include the Galaxy spanning Civil War?

Or, is the Horus Heresy portrayed as being a civil war but not involving chaos?
gpt
Horus as the image of the Arch-Heretic exists in the Imperial religious/history sources, and it's "known" that he was the Emperor's Warmaster.  It's part of the reason 'warmaster' is rarely used as a title by crusade commanders.  Instead, titles like "Lord Solar" are preferred.

He serves as an excellent warning story of what can happen when someone gets too big for their boots in the Imperium.  Likewise, the Emperor's Ascension Day is celebrated across the Imperium, though the specifics of what happened is likely obscured.

Horus' ties to the Warp, how close he came to winning, and the actual implications of the war are closely guarded secrets.