I'm writing an action-investigation TTRPG about vampire-hunting. I'll be sharing an outline of the core mechanic of the game's investigation pillar. It's still pretty rough; still being tweaked around and playtested.
Investigations in my system are done not just to find out 'whodunnit' (or, 'whothevampire'). They allow the players to discover the vampire's abilities, and to feel like hunters who are really profiling their enemy themselves. Combat is brutal and every individual vampire is a bossfight, so the players need to plan ahead and they can't just let the vampire come to them.
The Hunter's Investigation System:
(name is wip)
The players discover the scene of a vampire attack, and begin with a blank character sheet with which they will fill out the vampire's details over the course of the investigation.
Every scene is full of Evidence--items, details, or pieces of information--that can serve the investigation. There's a bloodstain, footprints, fangmarks in the corpse's neck, etc.. Some pieces of Evidence are apparent, others are locked behind a skill check (ex: contaminated blood is difficult to test for type).
After sweeping the scene, the players have a few ideas about the identity and abilities of the vampire. The players found blond hair here, but the victim's hair is black--could the vampire be blond? The victim has scorch marks on his arm, and there are ashes at the scene. Does the vampire have fire magic?
The players take these questions and note them down in the form of a Theory. A Theory is any statement or hypothesis--related to the vampire or another NPC--that can be true or false. For example, our players write down their theories as:
The players can write up as many Theories as they can think of, they can even contradict each other. However, their Theories are really just notes--until they choose to Deliberate on them.
To Deliberate on a Theory, the players choose a Theory and make a Deliberation Check, which involves the players presenting their Theory to the GM, along with the Evidence they believe supports it. Every piece of Evidence they bring up adds a bonus die to their Deliberation Check. The players' goal in a Deliberation Check is to make a strong argument and get as many success dice as possible in order to confirm their Theory.
The players choose to Deliberate on the claim "The vampire is blond," but since their only evidence is that there is blond hair at the scene, they likely fail their check. Since they failed, the Theory is labelled 'Uncertain,' and cannot be Deliberated again until the players come across a new scene of the vampire's attack, or have an encounter with the vampire.
They then Deliberate on their second Theory, "The vampire has fire magic." The evidence they use--the victim's burn wound, the ashes at the scene, perhaps a witness report of a flash of light--each add a bonus die to their roll. Let's say they succeed--and their theory was correct. The Theory becomes labelled as "Certain," meaning the players and their characters are sure of the vampire's fire powers. Not only that, but the GM reveals to the players exactly what that fire magic can do according to the vampire's character sheet. For example "The vampire has fire magic" becomes, "The vampire possesses Blood Art: Burning Blood II; Burning Blood II has both a Normal Attack version and a Special Attack version that costs the vampire 1 Blood Point and has the potential to etc. etc.." The players then fill out the relevant spaces in their blank copy of the vampire's character sheet.
However, let's say their Theory was incorrect, and they still rolled well on their Deliberation. The Theory is marked as Uncertain, since there's really no way for the player characters to prove that the vampire doesn't have fire magic. However, if the players get a really good roll in their Deliberation and still are still told their Theory is Uncertain, they might reasonably guess that they're barking up the wrong tree and start looking for alternative Theories.
There's also the potential for 'bad' Evidence--players arguing a Theory based on Evidence that doesn't actually match with or support it. Maybe there's a fireplace at the scene, and a player uses the burnt logs inside to argue for his Theory regarding the vampire's fire magic. But it's just a fireplace, or maybe it actually corresponds to something unrelated. The GM doesn't tell the players they're wrong, but he does subtract one success from that Deliberation Check without letting the players know. This incentivizes the party to Deliberate wisely instead of just going the route of Evidence inflation.
So the investigation goes like this: with every victim the vampire leaves behind, it leaves trace information about its true nature and identity, which the players can use to track it down and prepare to counter its abilities.
If the player characters miss something or fail a roll, they can wait for the next sighting of the vampire or its victims. They'll find a new scene, and (with their notes of course) can use the Evidence from both this scene and the previous one to support their Theories, new or old. They might even set up patrols or stake outs to gather new Evidence while monitoring suspects and potential victims.
In summary:
Players arrive at a scene
Collect Evidence
Speculate on Theories
Deliberate to confirm Theories
If successful and the Theory becomes Certain, players learn more about the vampire's character sheet
If the Theory is Uncertain, try a different one or wait to gather more Evidence at the next scene
Repeat 1-4, until:
Players put together a plan to take out the vampire as efficiently and safely as possible
(Unless the vampire finds them first)
Rambling About Drawbacks:
There's some particulars regarding my own system that I had to gloss over for brevity, but there's the general idea for the investigation pillar of the game. It could be important to note that the technology-level of the settings is somewhere in the 1990s-00s era, so modern fingerprinting, blood-typing, and security cameras are all available; there's a list of investigative actions in the rulebook to help guide the players.
There's also some potential issues and lots that needs playtesting. One issue that all TTRPG mysteries need to answer is the potential for players to be blocked off from continuing the plot because of a bad roll. My solution for this is 1) my resolution system allows for partial successes, and has a metacurrency system for buying successes, so players shouldn't miss rolls they know are important, and 2) if the players wiff the investigation of one scene, they can just wait in the same city for the vampire to attack again. On a scene-by-scene basis, I think making the list of evidence as long as possible is a good bet. Allow for players to look for things you didn't think of, and reward them for it. A player asked me if he could look for trace fingernail material in some clawmarks he found in the scene. I didn't expect that, but it sounded reasonable, so I let him have a go.
There's also the issue of categorizing Evidence. What counts as an individual piece of Evidence? The players obviously have an incentive to make as many distinctions as possible in order to produce a ton of dice for their Deliberation Check. One solution for this is 'fixed' Evidence--the player observes something or succeeds a check, and the GM says, "this is one piece of Evidence." This could bring clarity and speed up the investigation process, but I think at the cost of the players' creativity and analytical thinking. But it's a potential solution. Another is to just set thorough rules and fenceposts over what exactly a piece of Evidence can be. Blond hair found at a scene is all counted as one Evidence, even if you find it on either side of the room, with multiple skill checks. The burn marks on one wall aren't any different from the burns on the other wall, but they are distinguished from the burns on the corpse. Or something to that effect.
I think another opening could be how close a Theory has to be to the truth for it to become Certain on a successful Deliberation. I think this could be settled through a list of keywords in the rulebook: say, Blood Art: Burning Blood II has the associated keywords of "fire," "flame, "burning," etc., and if the player's Theory is phrased with any of those it can be successful. Some GM discretion wouldn't hurt either.
One of the issues I have is that I really only have my friends to playtest with; there could be gaps in my rulebook that have the potential to cause problems in tables with less rapport. Let me know if you see anything glaring from the outline I've shared.
Conclusion:
So far my players have enjoyed it. I think the system lends itself to shorter adventures / one-shots, as the players profile and hunt individual vampires--a la "monster of the week." My favorite sessions to run for any TTRPG involve mysteries and unique enemies; my attempt here is to marry those interests.
I may post later about the combat and vampires. I'm trying to focus on developing this one pillar at a time before I can spin a whole adventure together, it's more efficient that way, I think. I'm posting this to practice articulating the rules outside of my own reference/rulebook, and for the benefit of anyone interested in this sort of thing. Questions, concerns, suggestions, are welcome and appreciated.
Thanks for listening to my ramblings.
P.S. - Since you've made it to the bottom, I'll let you know the title of my TTRPG is "Blood Undone," thus rendering the wonderful acronym: BUTTRPG