I smile at the joke Graham probably missed. Appendix 7 is the material on running insanity in a game character.
The Multiverser cosmology asserts that there is a single Creator God and a multitude of created gods. Most religions that teach the existence of one single god are ethical monotheisms, and rather than create a lot of trouble over which one is correct we assert that (for game purposes) all ethical monotheisms worship the same Creator God, the differences betweent them existing because these religions are "close enough for mortals". Any religion that is not an ethical monotheism (Christianity, Judaism, Islam, Mormonism, I do not know that this list is complete) is worshipping a created god. Created gods are powerful because they were created powerful and given authority by their Creator to rule within their own spheres of authority. Thus we suggest that there is at least one universe (and possibly many) where Zeus was given authority to rule as head of a coalition of gods we call a Pantheon, each of whom was given authority over certain aspects of that universe.
Because gods exist in the supernatural realm and the supernatural realm borders on all natural realms, all gods can be contacted from all universes, with the caveat that whatever the ruling authority of that world determines to be the appropriate level of such interaction between the mortal and spiritual realms applies to all deities. It does not apply exactly equally, in this sense: if the power is predominantly good, the powers of good have the edge, and if the power is predominantly evil, the powers of evil have the edge. It is also possible for the predominant powers to be unaligned, in which case unaligned powers have the edge. ("Good" and "evil" here are used as shortcut terms for what is technically "alliance" and "anarch", respectively. "Alliance" powers are doing their best to serve the Creator; "anarch" powers are opposing the Creator; as mentioned, "neutral" powers have not (or not yet) chosen sides.)
If you invent a deity which you believe is a deity you invented, and then you worship that deity and call on him/her for magic, one of two things will happen. It is possible that nothing will happen, because there is no such deity. It is possible in the alternative that some spirit will consider this an opportunity to extend its influence, and will pretend to be the deity you invented. Probably over time it will attempt to persuade you that you did not invent it, but it revealed the truth about itself to you, although you may have gotten a few things wrong. In any case, if you seek to use holy magic, you are in essence requesting divine favors from the named spirit, and if the named spirit grants your requests, you own favors in return. You can't know how much those favors are worth until the other request comes.
Because it is more likely that an anarch power would want to deceive you into thinking he is the god you imagined, it's pretty dangerous to follow gods of your own imagination.
--M. J. Young