They cannot be killed but their aim is to abandon Earth, and they only need Aragorn to liberate them after they have fulfilled their oath. Not that Aragorn has

They cannot be killed but their aim is to abandon Earth, and they only need Aragorn to liberate them after they have fulfilled their oath. Not that Aragorn has true power over them. He has only the power to set them free, and they obey because that is the way to freedom.