From GameWatcher: “Stellaris: Nemesis lets you safeguard the entire galaxy against crises by taking on the role of a Custodian. But once you’ve cleared out any universe-ending threats, you have the option of holding onto your powers and playing out some players’ ultimate authoritarian fantasy by establishing the Galactic Imperium.

Naturally, if you’re the one who lead the charge against whichever crises threatened everything, you also get to sit on a hopefully cozy throne and order everyone around, at least to some extent. Proclaiming the Galactic Imperium sees all existing federations in the Galactic Community disbanded and its member being absorbed into the Imperium itself.”

Source: N4G PC Saving the Galaxy in Stellaris: Nemesis Lets You Establish and Lead the Galactic Imperium