This article is all about running a Call of Cthulhu game involving the Unspeakable Promise in the World of Darkness system, though there's some advice here that's good for people running the game with the BRP or Trail of Cthulhu system. If you'd like to read more articles where this came from, you can find them over here.
The Unspeakable Promise
Some people are foolish enough to make bargains with Hastur in the vain hope that what they gain will be more than what they will lose. In the fabulous Tatters of the King campaign (by Chaosium), your characters not only get the chance to deal with Hastur but also have the opportunity to learn the spell themselves and could be foolish enough to make the promise. How do you deal with that?
Firstly, try not to be too much of an evil genie about the promise. Hastur is far more insidious than that. The promise will work. The characters will get what they asked for. After all, there wouldn't be so many Hastur cultists willing to make the deal if it never worked out. Just look at Edwards and Bacon who both seem pretty happy with what they've gotten. If an investigator makes a deal to become an epic marksman, they really should have spells or skill buffs that make them a better gunman.
Secondly, be a bit of an evil genie about the promise. Whose to say that Edwards' homicidal sadistic / masochistic tendencies weren't a byproduct of his deal for the ability to regenerate himself? Whose to say that the cost for Bacon's power and knowledge isn't the need to drain life from victims (despite regular usage he's still not all *that* young) or that he wasn't turned into a sociopath from the sudden insight? Have a think about what the cost will be for the deal made.
Let's return to the marksman idea. Perhaps the character becomes increasingly fixated on figuring out ways to kill the people around them. You don't need to tell the player to become fixated. You could simply start describing good sniper placements, fields of view, possible exits, alongside lovingly described depictions of their enemies dying and just how easy it would be. It might begin with villains but then expand to include police officers who give them speeding tickets and then annoying waitresses.
In the Storyteller system, you could use the vice mechanic to highlight the descent into the darkness. Treat killing someone with a gun as an extra vice for the character. Remove the ability to make Morality rolls to avoid losing Morality when the character murders someone with a gun. They should automatically lose it. In the BRP system, you might give them a 1/1d6 sanity check whenever they fire their gun due to the shocks of pleasure and joy it gives them.
Hastur is all about madness, after all. Drive the characters mad but do it through temptation.
The Unspeakable Possessor
When a character who has made the Unspeakable Promise finally dies, they are possessed by Hastur itself (or perhaps some sort of lesser aspect of Hastur) which causes the victim's body to twist into a boneless giant humanoid with an overwhelming desire to kill others with its draining touch though it is capable of animalistic cunning. This isn't an entity that you would want to face lightly.
Additional skills may be retained from the original host at the Storyteller's discretion.
Le Parkeur: 4
Armor: 3/3 (scales and rubbery flesh)
Draining Slam. On a successful touch attack, the Unspeakable Possessor rolls Resolve + Occult versus the target's Stamina + Supernatural Potency. If the target fails this roll, they die. If the target succeeds on this roll, rather than dying outright they take aggravated damage equivalent to the successes made on the original Resolve + Occult roll.