[Stormcrow]

In the old Fantasy Wargaming book, everything supernatural—magic, gods, demons, spirits, everything—was based on mana and the Ethereal Plane. The more you believe in a supernatural entity, the bigger and more powerful that entity becomes. During the height of ancient Greece, the Greek gods were worshiped, and worshipers' ceremonies transferred mana to the gods, making them powerful. As worship of the Greek gods faded, so did their power, until they became minor spirits hardly able to do anything. As people stopped believing in the elves of medieval legend, they got smaller and smaller until they were Victorian pixie-fairies. Even the Christian god demands worshipers, whose worship transfers mana to him for his existence and use. (In this model, the Christian god being a jealous god is very literal: he suppresses the belief in other gods to keep them from getting as powerful as he is.) Mana is also the medium of magic; wizards perform rituals to generate mana which they keep for themselves to use in magic.

In GURPS, you can do the same thing with character points. Belief in a supernatural entity generates and transfers character points to that entity. Come up with some kind of formula or table that shows a relationship to an Ethereal entity's current level of belief and the number of character points he has. Magic may not be powered by character points, but an entity's level of Magery certainly is. And so on.