How to Create Deities (3.5e Guideline)
From D&D Wiki
This is a quick reference guide for individuals on how to create deities. This will help you assign what divine rank they are, which grouping they should be within, how you would classify them and finally the information that should be present throughout the article to make it worthwhile.
Rank 0 aka the Hero ranks are going to be full of individuals who have been given a trace of god hood. In life famous examples would be Hercules, Perseus, Achilles etc, etc. They might have a following in rare instances but this would be considered unusual and the big thing that sets them apart is that they are effectively extremely high level PC class characters with a single outstanding quality (though rarely two might be justified.) Unlike a true deity most Hero class deities will not have the 20 levels of outsider and are generally a lot easier to kill (Though the term is realitive. For example Hercules is known for strength and to a lesser extent constitution; even though he doesn't have the natural armor and other bonuses he would still receive the full hit points for each level and probably rival or perhaps exceed most dragons for the ability to receive punishment.) Further these individuals will have the base bonuses associated (DR 10/Epic, SR 32, Fire resist 5, etc.) One thing everyone seems to leave out is that Hero deities at a minimum need a stat line, as these should be able to be defeated.
Rank 1-5 aka the least deities and demigods. These have very limited and small abilities to affect other planes, generally have very few followers and generally are not insanely powerful. Most of these gods (while getting a large boost over the Hero level divine ranks) are actually on par with sub generals on the planes of hell or some of the lesser powerful gods of the abyssal layers. If a god has a small specialized subset then he will normally work within this group of rankings. Example: If a god is worshipped in a single valley in a world (of three to four villages with medium populations) this grouping would be most appropriate.
Rank 6-10 Lesser deities are able to perform more acts in the realm of true miracles and generally will have medium sized congragations. Their spheres of control are larger and more defined, and generally they will be on par for power with the controllers from the first three planes of hell or the majority of abyssal layer gods. These deities are most appropriate if the individual would be worshipped by a few major cities worth of people (generally region or kingdom wide, as followers won't people an entire town.)
Rank 11–15 aka intermediate or true gods. These are the large gods that will have many followers and generally stretch globally, though not necessarily penetrate globally in equal parts. Hundreds of thousands will call upon them for intercession, and prime examples from ancient times would be Ares and other gods of importance from the Greek pantheon that are not considered controller gods. Their spheres of influence are expansive and powers are great. Game examples would include planes 4-6 of hell, Lolth, and many other named deities of the Abyss.
Rank 16–20 aka greater gods or controller gods. Worshipped in droves world wide these deities possess and can influence truly impressive events. Strength from their followers allows these deities the ability to control large areas and produce miraculous results from divine energy alone. These gods will generally be at least known of throughout a world, even if different parts of the world might call them by a different name.
Rank 21+ aka over-deities. Over-deities are an enigma; they no longer require worshippers and work on a cosmic scale, not particularly involved in any particular plane because it is beneath them. Most individuals won't be able to even name an over-deity and likely the only way individuals would even be aware of them is through advanced and laborous research. The spheres these deities work over are generally more conceptual in nature such as good, evil, curiousity and any other nameable attribute.
First define for yourself what your particular god (or goddess) does upon your plane. Are they a god of a broad concept or a narrow concept (for example a god of justice is a narrow concept, a god of law is a broader concept and a god of order would be the largest concept.) Once you have defined their primary sphere of influence the next step is to establish if there are any additional spheres of influence that they also have control over (i.e. is he a god of truth and justice or justice alone?) From the initial concept you can determine what grouping to put him within, to determine his appropriate divine rank. Once you have the appropriate rank; how does this god compare to other gods? Is he closer to the next rank up or down in theoretical power? Is their an opposing god? All of these questions are vital to how a god becomes fleshed out enough to bring the original concept to the forehand.
When it comes to deities in almost all cases the more information you can provide the better. Where do they reside? What part of that place do they reside in? Do they regularly grant their followers boons, or are they extremely stringent with aid? Has any rivalry between themselves and another god spilled over to open warfare with the religions? The questions can easily become infinite, as gods unlike characters since they don't have a compressed timeline. While even long lived races such as elves might prove a daunting task to write a full backround for depending on age the backround of a god is no less important. Completeness of all regularly (and often irregularly known) information is essential to give the god life and bring a deity that acts in a correct fashion into the world.