Generally speaking, players are more likely to want to get involved if the cataclysmic event is closer to home. "There is a heresy building strength in your demesne." "An usurper is rallying an army against your citadel." "Rebellious elements are gathering to demand a lowering of taxes in your demesne." Leading police forces to punish things is something I would do. Trying to save an empire that is collapsing around them is something the owner of a single castle should not be responsible for.

Adding more fantastic threats does not make your game High Fantasy. The stakes are already high enough with the empire about to collapse and the known world about to be ruined. Adding more things doesn't change the fact that the world is ending. Low Fantasy is about smaller things like farmsteads, mines, trade routes or villages being in jeopardy. High Fantasy is about world-changing events. Adding in fantastic stuff doesn't change the stakes.