If the unwelcome visitors are a threat to your entire fictional world, how did this come about? Where are they from? What are the historical clashes between that world and the one you write about? Equally if they're a new threat, what made them decide to pick on your world? What assets does it have the unwelcome visitors may want to steal?
On an individual level, who do your unwelcome visitors pick on and why? Have your "victims" done anything to deserve this? How are the unwelcome visitors got rid of again? What trouble do they stir up? How would your fictional government deal with unwelcome visitors who threaten their existence? (That kind of visitor is nearly always after a complete take over and they're unlikely to pension the previous regime off!).