AND make it satisfying for you as a player?
Let's say the group has an NPC ally/enemy/frenemy that has danced back and forth between LN and LE throughout the campaign. Born and raised in Cheliax, affably evil, and fairly sympathetic, motivated primarily by love/loyalty to family. While most of her retinue could more easily be swayed away from evil(if they ever were to begin with), this character would stay hovering right at the edge of LN/LE at best for the most part, with better moments that shine through every now and then. (try to imagine a much more sympathetic/less sociopathic Azula and her running crew as-is and you're not far off)
And then that character finally goes full-blown diabolist(as in the PrC from Book of the Damned) and sells their soul, either for power they want for themselves or their family/house or, to twist the knife further, to help those people they've allowed themselves to consider something close to friends.
The PCs would inevitably find out, and they could very well get an honest look at the actual fate in the afterlife that awaits that NPC, which the diabolist may or may not be truly aware of; they might very well be deluded into believing that they'll really have a place of honor in Hell rather than just being another brick in the wall.
So the diabolist either eventually dies during or after the campaign as a tragic ally or enemy, depending on how the relationship with the party holds, either accepting their fate or fighting futilely against it, or the party tries to save that character.
But how? The NPC knew what they were getting into, so the usual tropes that play into the "trial in Hell" stories don't really come into play. And if things do get to this point, it just seems to scream for a bittersweet outcome at best.
I think I have some ideas to throw out there if the players refuse to accept that chracter's fate and fight against it, but I was just wondering what approaches others might take to this possible plot thread. I'm kind of veering towards Planescape/Gaiman territory now, but that scope could easily overshoot that of the main campaign. Also considering the "equal exchange" or "what else can you offer?" route, which has to be costly and painful, even if it winds up being worth it in the end.
(this is all if's and maybe's at the moment, because so much will change depending on the players in both groups I'm running this campaign for, but y'know the Boy Scout motto: "Be prepared.")
IIRC there was some infos on devil's contract in Princes of Hell, basically each party (the devil and the contractor) got a contract and if both are destroyed, the contractor is freed. The problem is that the contract of the devil is stored in one of the vast Fortress-library of Hell. So some kind of "seek and destroy the artefact" campaign.
Man, I completely forgot about that standard presented in that same book! Thanks! It could work, but it just feels like it needs to be spiced up some more.
Admittedly, convincing the diabolist to destroy their half of the contract might a hell of an RP struggle all on its own.
Further details that could tailor the concept:
Eventually, some time after having met and interacted with the PCs for a time, she'll make her deal, believing she'll be granted a place of honor in her patron devil's court in hell. And some time after that either she or the PCs will discover that the fine-print-interpretation really means that she'll literally be a Wayne Barlowe/God's Demon-style brick in the wall of her patron's trophy room....right next to her mother. So now she has that extra temptation to just give up and accept her fate. It's at least one more hook Hell has in her.
edit-I think I just got a lock on what her final fate should be if she escapes her fate. It's a fitting "doesn't get off too easy" ending, I think; salvation and punishment all in one. Perdemption, one might say. Now it's just the actual doing that needs work.
Depends on exactly how the contracts work.
She could use a VERY carefully worded wish to make the Devil repent (or suffer whatever consequences for refusing his end of the contract), though the wishes have to be made in 24 hours so it's likely past that point.
Ah...the Grim Grimoire gambit. :D
It would likely be long past that point but that's still a nice option to keep open to help the situation.
While not exactly what you want something to think on and twist about would be calling on a god or another god as the case maybe.
Something else that occurs. You could somehow have something or something prevent some aspect of the contract to not be fulfilled leaving wiggle and bargain room for the damned.
For example: I have a mountebank. His terms were when he made the bargain as a child was that he would get to live and get to kill Lam. While he did live and was part of the operation that killed Lam be himself did not do the killing. It was the cleric of Cayden Cailean who did the killing.
What should it take to save a diabolist from damnation?
Direct Daeific intervention.