User blog comment:FairyRaven/Fairy Tail Chapter 425: Sabertooth X792, Review and Commentary/@comment-31.132.65.54-20150324135043/@comment-7418318-20150326174359

I haven't been thinking about the Shadow in quite some time.

With Skiadrum having been alive all the time it can't have been him and I think we can also rule out that F-Rogue was the shadow.

But I have another idea. Maybe this "shadow" is not an external influence, but something from within. But instead of being some inner demon, I think it might be the manifestation of a mental degeneration that happens during Dragonification.

Imagine if Acnologia used to be a good guy, but due to turning into a Dragon his mind was damaged and he went through the exact same thing as Rogue did, only he lost the mental battle and became the sadistic beast he is today.

That would also be a possible explanation of how F-Rogue survived. Rather than using his shadow magic to hide, Acnologia could have felt that F-Rogue was something close to being kin and thus intentionally spared him and waited for the process to run it's course. After 400 years of loneliness the prospect of having an equal and comrade might be tempting to even the worst kind of beast.