Endgame Factions are different factions that track the progression of certain game systems. They can be found in the factions panel (bound to Y by default).
Unlike Item Factions where you can only be aligned with one of them per character at once, Endgame Factions are not mutually exclusive. You can belong to an endgame faction and an item faction at the same time, and you can belong to multiple endgame factions at the same time.
Different Endgame Factions may provide different rewards and challenges from others, and their progression isn’t always as simple as just slaying enemies or gaining and spending a specific currency tied to them.
Forgotten Knights is an Endgame Faction that guides you through the progression of defeating the Harbingers that you will encounter in Monolith of Fate.