What are denizens and what do they do?

They’re emissaries of Skaia, final bosses tailored to either the Hero’s Aspect or something else, meant to pose a challenge they must overcome with their Choice. Either to prove the grit and determination of the Hero, or, were they to fail, to test their Strength in combat. They rule over the Land and create the Underlings at the behest of the Dersite Agents, and stand as the Ultimate Challenge the Hero must overcome before reaching the Endgame and having to go face the Royalty. Or perhaps as the Ultimate Challenge AFTER defeating the Royalty? It seems going for the Denizens should be a step before going for the Black King, but you may actually be able to go do stuff with the Denizen afterwards.