Introducing new spells

The question of how to introduce new spells into a game came up today in one of the online gaming forums I frequent. For example, when you introduce a cleric spell from a new sourcebook, or when a cleric PC researches a completely original spell, do ALL clerics suddenly gain access to it? I replied there, but decided to share (and expand on) my answer here.

I personally like the rules for "uncommon divine spells" that appeared in the introduction to Pathfinder Short Cuts #2: Inquisitor Spells of Freeport, by Owen K.C. Stephens (PDF available in the Green Ronin Online Store). While written for Pathfinder, this concept is easy to adapt to any d20 game where distinctions are made between casters who get full access to their class's spell list (such as most divine casters) vs. those with a limited number of spells known.

The general idea is that when introducing new sourcebooks into a campaign, it can be very unbalancing to allow divine spellcasters to immediately gain access to all new spells included in that content. In addition, the need to reference multiple books can bog down the process of daily spell selection. Instead, all spells outside the core rulebook are considered to be uncommon, and require the caster to find and study a description of the spell before they can learn it. This process is similar to how a wizard learns new spells, but is less involved (and less costly). Divine casters who already have a limited number of spells known (such as oracles and inquisitors in Pathfinder, or rangers in 5E) ignore this restriction.

[I use this rule in my "Time of the Tarrasque" Pathfinder campaign. Our only prepared caster PC is a cleric/wizard aiming for mystic theurge, so making the process of expanding her divine repertoire mirror that of her arcane class is perfectly on-theme.]

In contrast, the arcane classes must all specifically learn each new spell before they can cast it, and (in 5E) only the wizard can learn an unlimited number of new spells. Therefore, you don't really need any new rules to limit these classes' access to spells from new sources. However, the idea of core rulebook spells being common, and spells from all other sources being uncommon, is still handy for explaining why a new spell (for any class) isn't as widely known in the world, and why it might take more effort to track down the means to learn it.

Finally--and I hope obviously--any original spell created through the spell research rules is automatically an uncommon spell. At first, other casters will have to bargain with the inventor to study her original notes before they can learn to cast the spell. As more people learn it, finding a teacher becomes less of an obstacle. Of course, many spell inventors will jealously guard their secrets in order to reserve the advantage for themselves--and sometimes their allies.