Gotta say that @EmperorKinyagi is correct in that there's usually a hidden final boss that's never promoted.
The promoted boss and the final boss tend to be on the opposite ends of the spectrum. Artorious made it seem like the dark is truly horrific and needs to be suppressed. Manus proved the dark is the true nature of humans and completely natural. Maria and the Orphan of Kos were similar.
Based on this, Messmer would paint something as bad or in need of suppression. The final boss would do the opposite and odd lots of context to the base game story leading up to the endings.
The promoted boss and the final boss tend to be on the opposite ends of the spectrum. Artorious made it seem like the dark is truly horrific and needs to be suppressed. Manus proved the dark is the true nature of humans and completely natural. Maria and the Orphan of Kos were similar.
Based on this, Messmer would paint something as bad or in need of suppression. The final boss would do the opposite and odd lots of context to the base game story leading up to the endings.