Spheres of Influence (CPrH)

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As you'll recall (from the DMG, pages 33, 34), priest spells are divided into categories called spheres of influence. Each spell belongs to one of 16 categories. Those categories include:
All, Animal, Astral, Charm, Combat, Creation, Divination, Elemental, Guardian, Healing, Necromantic, Plant, Protection, Summoning, Sun, and Weather.
Priests can't cast spells from all spheres of influence. Any specific priesthood can have major access to one or several spheres, and thus eventually learn to cast spells of any level from that sphere, and can have minor access to one or several other spheres, and learn to cast spells from only 1st through 3rd level in that sphere.
When designing a new priesthood, the DM decides which spheres of influence the priesthood has.
All priesthoods should have major access to the All sphere. Beyond that, the DM should choose a number of spheres, and the access to each, based on the attributes of the god being served, and on how combat-efficient the priest is already.
If a priesthood currently has access to good armor and a wide range of weapons, the DM should limit the range of spells available to them. If, however, the priests do not have access to mighty engines of war, the DM may want them to have a greater ability with magic, reflected by access to more spheres, and major access to a greater proportion of them.
Priests should have Major access to just about any sphere that has any bearing on the primary attribute of their god, and at least Minor access to spheres which have a lesser relationship to their god. (For instance, priests of a God of War who is noted for his protectiveness might have minor, or even major, access to the Guardian and Protection spheres.)
Here are some rules of thumb to go by when assigning Spheres of Influence to specific priesthoods:
==The Priesthood Has Good Combat Abilities==
If a priesthood is allowed to use metal armors and a good range of weapons, we consider that it has good combat abilities. Therefore, it should have less effective magic, including:
 Major Access To: The "All" Sphere, and two other Spheres.
 Minor Access To: Two Spheres.
==The Priesthood Has Medium Combat Abilities==
If a priesthood is allowed to use metal armors but restricted to a poor range of weapons, or is not allowed to use metal armors but has access to a good range of weapons, we consider that it has medium combat abilities. It should have a more average access to magic, including:
Major Access To: The "All" Sphere, and four other Spheres.
 Minor Access To: Four Spheres.
==The Priesthood Has Poor Combat Abilities==
If the priesthood is not allowed to use metal armors (or even any armors) and has access to a poor range of weapons, we say that it has poor combat abilities. It should have an enhanced access to magic, including: 
Major Access To: The "All" Sphere, and six other Spheres.
Minor Access To: Six Spheres.
==Some Definitions==
"A good range of weapons" is not a very precise definition, but it's harder to be more precise; what is and is not considered a good range varies with each individual DM. In general, if the priesthood is allowed access to five or more different types of weapons (a bow isn't that different from a crossbow for our purposes, but a mace is different from a spear), or if the priesthood is limited to a few efficient and high-damage (1d8 and better) weapons (such as swords), we say that it has a good range of available weapons.
"A poor range of weapons" is just as imprecise. In general, if the priesthood is allowed access to four or fewer different types of weapons, or has access to a greater number of weapons which do no more than 1d6+1 damage, we say that it has a poor range of available weapons.
If a priesthood doesn't allow the priest access to the full number of spheres appropriate to that type of priesthood, then the DM should supplement the priesthood with extra Granted Powers. For instance, if a priesthood has Good Combat Abilities but is designed with major access to the All sphere and only one other sphere, and minor access to two spheres, then the priesthood is receiving less abilities than it should; it should be given some minor Granted Power to compensate.
Now, this valiant effort to keep combat abilities and magical abilities balanced is substantially thwarted by the original Cleric class himself. The Cleric has major access to 12 Spheres, minor access to one, and the ability to wear metal armors. In short, he is more powerful than just about any more restrictive priesthood.
If the players in a campaign are likely not to take priests of specific mythoi simply because the original Cleric class is more powerful, the DM may wish to revise the Cleric in his campaign. For more on this, see "Toning Down the Cleric" in the Role-Playing chapter.