Clerics are all messed up.
I run a lot of classic style (basic/expert and 1st edition) games and I don't use clerics.
Problems include people feeling like they need to take a cleric, a complete dissociation with how polytheistic religions actually worked, and questions about how society looks if it can eradicate disease and raise the dead.
If I do have to run clerics, I follow the advice located in Men and Magic.
"The number in each column opposite each applicable character indicates the number of spells of each level that can be used (remembered during any single adventure) by that character."
What if it meant exactly what the meanings of the words implied it means? (What are the chances, right?)What if spells can only be used during any single adventure.
The cleric gains her spells because she is seeking out danger for her deity. If she stays home, she is granted no spells. Only when she leaves home to defeat evil does she have access to her magic.
But mostly I don't use them.
Devotions
I replace clerics with devotions. There is no cleric class. Any person may devote themselves to any deity. Each level of devotion takes 5% of the characters total experience. Devotions run from levels 2-12. After each adventure, roll 2d6, on a roll equal to or higher than the devotion the character can gain levels in their devotion. Each new level grants a new power.
The roll after each adventure is not entirely random—Gods come with a variety of observances and taboos. Following the observances of your deity will put you in their favor and allow you to become a more powerful servant. They also have forbidden activities known as taboos. Breaking a taboo immediately causes your devotion to be reduced by 1. If your bond is reduced, you lose access to the powers you gained as you fall out of favor. 2d6 is a bell curve roll meaning that devoted worshipers will rise in the god's esteem faster.
This directly engages the character into performing appropriate behaviors for mechanical benefit, and allows any class to add devotion to a god to their abilities in exchange for experience. Rules like this drive emergent play. This also dovetails with the ability to create congregations, which provide monthly value in xp/gp, allowing those who maintain a flock to subsidize the devotion expense with the value of their worship.
Example
Boccob, the Greyhawk Deity of Magic, the uncaring, the lord of all magics, and the archmage of the deities.
Taboos:
Destroying books, magic items.
Observances:
Acquiring new arcane knowledge (+1)
Engaging in magical research or experimentation (+2)
Burning Incense (+1)
Creation of a magic item or spell (+4)
Utilizing florid and elaborate language (+1)
Devotion level 2: Detect magic at will
Devotion level 3: Gain the ability to cast comprehend languages or command 3 times a day
Devotion level 4: Gain your devotion level as a % bonus on all magic/spell research/creation rolls.
Devotion level 5: Gain +4 on any saving throws versus spells
Devotion level 6: If you cannot cast spells, gain a spellbook and the ability to cast spells as a 1st level wizard. If you can cast spells, treat your character as one level higher in regards to spells slots available.
Devotion level 7: Once a day you can reflect any spell targeting you on a successful saving throw versus spell.
Devotion level 8: For the purposes of crafting magical items, golems, scribing scrolls and brewing potions, your class level is considered 5 levels higher. (For campaigns that require a minimum level to craft—for a more modern game, I would grant a cost reduction or crafting time bonus).
Devotion level 9: You can cause your nails to secrete a magical ink that can be used to scribe spells and scrolls for free.
Devotion level 10: You are considered blessed by Boccob and your intelligence increases by 2.
Devotion level 11: You can overcome the resistance of bindings and metal better than normal arcane wielders, Armor up to chain can be worn without affecting spellcasting.
Devotion level 12: If you can only cast spells because of your devotion to Boccob, you are now considered a 5th level caster. If you can cast spells from another class, treat your character level as two higher in regards to spell slots available.
This makes worshipers of deities in a realm where the gods are real into real worshipers, and creates impactful issues in the day to day lives of the player's characters. Now any character can follow as many gods as they want, as long as they are willing to spend the experience points to do so. The scope of work is limited, needing only to cover any deities you consider important and any the players select.
Here's A guideline for creating Devotions from deities.
Consider taboos—any of which will cause the characters to drop a devotion level immediately
Consider observances—These should be material, actionable acts within the game that will come up during the course of play. Consider activities pre/post camp/battle, behaviors and traits of the deity and the ways those can be propagated by the players.
Designing the Devotion levels.
Devotion level 2 should be a minor constant bonus or other baseline cantrip level ability relevant to the deity.
Devotion level 3 should be a limited use signature ability, that will characterize the worshipers in society.
Devotion level 4 look to a non-combat/non-adventure ability relevant to the gods. It should have some utility.
Devotion level 5 Should be an appropriate defensive bonus or ability
Devotion level 6 Should be a spell or power that can be used at will. You should take the ability and integrate it into a ritual or effect. A enchantment spell might include coating a tongue in quicksilver, a firebolt should require a flaming pellet, a divination spell might require a sacrifice or anything
Devotion level 7 This level should contain a thematic deific ability. A storm god could cast a lightning bolt, an animal god might allow shapeshifting, or some other key ability related to the gods domain. At this point, the character is giving over one third his earned experience to the deity and this power should be for that exchange. Devotions higher than this level are more difficult to get due to the bell curve on 2d6, progress beyond this point will be less quick.
Devotion level 8 This should be a unique, chance based ability, or one that has utility that isn't directly related to combat or power. It should be active, but a constant bonus, such as increased hit die to undead raised, a bonus to some skill or class ability, and something that brings the deity into the fabric of the world.
Devotion level 9 This level represents the deity expressing their glory through the character. It provides some utility and a visible transformation.
Devotion level 10 This level should be a passive blessing or bonus that indicates the grace of the possessor. It may or may not also have a visual component.
Devotion level 11 This level should provide a major power or benefit of the deity.
Devotion level 12 It should be noted that level 12 is fairly difficult to achieve. It requires a roll where the total is higher than 12. Even with a bonus of +2 from observances, there is still only an ~9% chance. It is unlikely in all but the longest campaigns that characters will realistically reach this level, therefore it is ok to provide an appropriate capstone ability for the deity.
Characters should advance fairly quickly to the median of the bell curve (level 7 or 8) and then will progress more slowly as it becomes more difficult to score the higher rolls.
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