Interesting idea!
First, I'll brag on an old LARP I ran, then I'll give you an idea
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This LARP was centered around a cabal of modern-day occultists who unintentionally summoned a demon into the world. Few of the cultists had any real magical power; one of the only ones who did was so traumatized by the summoning that he became an amnesiac. When he awoke, all he had with him were a deck of the major arcana from the Tarot.
We made up 22 cards for the player, set up like a taped-shut greeting card. On the front of the card we put a scanned-in design of the tarot card; beneath it, we wrote a hint as to the card's power. Some of the hints were cryptic: "Start them anew," said the Fool's card. Some were straightforward: "Brings Light," said the Sun's card. The devil's card said, "This card terrifies you: use it only if you have no other choice."
When the player activated a card, he opened it to read the effect. "Show this card to your victim," said the Fool's card, "who will forget everything that has happened since he first saw you in this scene." "The room fills with glowing natural sunshine," said the Sun's card, "and will stay that way for five minutes."
The Devil card's power was based on Aleister Crowley's devil-interpretation: self-knowledge. When he opened the card, he found a GM for the game, who gave him back his memory. It was great fun.
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Ahem. I was supposed to be giving you ideas, right? Let's see.
What if someone is doing a massive reading with these people? They've offered these tattoos to people under some sort of pretext: perhaps the tattoos have a minor magical power associated with them. However, the tattoo ink is also poisonous, and it kills its victims at a random time. The murderer scries on the victims and sees where they fall; by reading the correspondences between the victims, the murderer is able to perform a grand divination, perhaps predicting the time and location at which a portal can be opened between worlds.
The victims should either be insignificants, like sailors or orphans, so that their deaths won't be heavily investigated; or the tattoos should be illegal or otherwise shameful, so that their survivors won't be willing to go to the authorities with information about the tattoos. The latter scenario might be more fun: perhaps the tattoos contain a good amount of power but also contain symbols of an illegal religion, such that the victims are willing to be tattooed but have to keep it secret. Then you can choose victims aligned with the cards: a cleric for the hierophant, a lycanthrope for the moon, a Siamese-twin for the lovers, and so forth.
Consider having a cabal responsible. The cabal ought to involve some sort of Tattoo artist PrC (Relics and Rituals has one, I believe, as does the Book of Eldritch Might, or you can just make up a feat called Create magic tattoo that works similarly to create wonderous item). There should also be some sort of diviner -- Relics & Rituals 2 has a High Astrologer that would work, or you could choose something from Tome & BLood, or you could just have a wizard who specializes in divination.
Ideally, the characters will be called in to investigate the first murder, will hear rumors of other mysterious deaths, will begin to piece things together, will hear of another death happening, will consult someone who can explain that there are 22 cards in the major arcana, will figure out that 21 people have died, will realize who is probably going to be the 22 victim (who should be someone politically very important), should have to race to save this last victim, and should be confronted by cultist mooks during this effort. If they succeed in stopping the 22 death, they'll need to track down the cult before it can try a different scheme; if they fail, they should get one last chance to find the cult before it does whatever action the Tarot Corpses pointed them towards.
Daniel
