Setting: The game world must have little to no divination magic. Either it never did, it was outlawed, it was lost, or it requires so much energy, that few bother to learn it. Also, any spells that give precise information must
a. be toned downSpells like detect alignment only reveal vague information. Detect evil only lasts as long as the caster could concentrate and only for a number of rounds equal to a Concentration check (hey, that’s a good rule anyway). Locate object and locate people only point in a single compass direction (North, Southeast, etc.). The closer you are, the more precise the information, but you still need to know exactly who you are looking for, etc. Speak with dead has to go. Otherwise every murder would be solved in an instant. Stonespeak (or whatever it's called) would also be limited and speaking with animals would give just vague answers like “human”, “orc”, “rhino”, and “rodeo clown.” Perhaps a squirrel can identify gender, but I doubt it... unless another squirrel committed the murder.
b. be removed from the game
c. require massive expenditures of Constituion, hit points, XP, or material components
Rules: Whatever race dominated the world would be the majority of races prevalent in any CSI team. Of course, the token black elf would make an appearance to improve ratings, but the king is going to want people he can control. Everyone would have to be lawful, of course. And their allegiance would be to their country first.
Skills like Diplomacy and Intimidate become very important and a more precise method of interacting with people becomes necessary. Willpower becomes extremely useful for both PCs and suspects… the characters are going to get tired of asking the same questions over and over again.
Gather Information is also too broad of skill to just reveal (Bob the Bartender saw her last). The PCs would need a new way to interact beyond the present scope of Dungeons and Dragons (which, sadly, has no real social measure).
More on this in another post.
World Logic: Beyond what was stated above (cf. divination magic), all other logic is at the behest of the DM; although, in a CSI campaign, the majority of bad guys should be humanoid. A beholder that murders someone isn’t really within the realm of “investigation” the same way a guild assassin would be.
Missions: Adventures could be episodic (a new murder every week), or a single campaign could focus on tracking down a single serial killer, with or without short murder mysteries in between. Railroading is an absolute NO-NO in a murder mystery. The clues and facts of the case must be designed by the DM ahead of time within enough room for misinterpretation and red herrings.
I think Robin Laws once wrote that murder mysteries are the hardest adventurers to design, and he’s half-right on this. Because the PCs may never get the clues or the clues may be too obvious, it is important to write out 20 small clues as opposed to 3 big clues for the PCs to find.
Magic of course makes some of this easier to uncover. And of course, finding traces of sulphur on the scene means that magical fire was probably used on the victim.
DM Advice: Keep the campaign focused on the prize and the PC’s interaction. Most CSI-style shows are built around 50% methodology and 50% banter. Make sure the created PCs are interesting and even persuade the players to choose “actors” that might portray their characters.
I’ve run a similar campaign before and I know this works. BUT, it requires the players to come to the table with similar understanding and expectations about the campaign. You will never be Krull the Deathdealer in this campaign. Come up with something else.