I’m a fan of the d20 System not only for its Open nature (the OGL is, I firmly believe, a truly great thing), but also for its variability. It’s really a system that can handle almost anything, though with the caveat that you’ll sometimes have to change it to some degree in order to make it really do what you want it to do.
It’s that line of thought that brings me to today’s point. Lately I’ve been reading a lot of urban fantasy novels that have the protagonist acting as a sort of “magical sleuth,” hunting down criminals via magic (bonus points if you can guess which novels). Often, this means that the criminals were using magic themselves, and a lot of the detective work involves figuring out what spell it was, how it was used, and why the person cast it.
For a while, I’ve been wanting to try and recapture that same sense of detective work in a Pathfinder game. The problem is, strictly speaking, that investigating an incident of magic after the fact is very difficult. The detect magic spell does let you examine auras for spells after they’re cast, and determine the spell’s school…but even these have problems. Namely, that spell auras linger for a very short time, and simply identifying the school of magic often isn’t enough.
Hence, I’m introducing some variant rules here, to try and make detect magic more viable in terms of investigating after-the-fact spellcasting. The following replaces the listing for the “3rd Round” entry of the detect magic spell description:
3rd Round: The strength and location of each aura. If the items or creatures bearing the auras are in line of sight, you can make Knowledge (arcana) skill checks to determine the school of magic, and even specific spell, involved in each. (Make one check per aura: DC 15 + spell level, or 15 + 1/2 caster level for a nonspell effect – if you exceed this DC by 10 or more, you identify the specific spell; see below for details.) If the aura eminates from a magic item, you can attempt to identify its properties (see Spellcraft).
A character that beats the Knowledge (arcana) DC by 10 or more learns the specific spell that created the aura, if they are familiar with that spell. In general, characters are familiar with spells in their spellbook or on their spells known list (for arcane spellcasters), or with the spells from the Core Rulebook that are on their class spell list (for divine spellcasters), of a level they can cast. Characters who identify a spell that they are not familiar with gain a general (i.e. one-sentence) description of that spell. The GM has final say over what spells a character is and is not familiar with.
Similarly, the following table replaces the one found in the detect magic spell description for how long lingering auras last:
Original Strength Duration
Faint 1d6 x 10 minutes
Moderate 1d6 hours
Strong 1d6 days
Overwhelming 1d6 weeks
These changes allow for more time and more accuracy in allowing characters to determine what magic was used somewhere. This, in turn, is likely to make characters more hesitant to cast spells, knowing that they can be identified later (and also thus more likely to try and find/create/purchase methods of defeating this identification).
While there’s more to a magical-mystery game than this, hopefully these alternate rules are a good first step to letting you run a CSI: Golarion adventure.