Tomorrow's one-shot game, "A Heresy of Angels" was marked long ago as containing "mature themes", a term which I've never quite tied down.
Now, it's perhaps worth noting at the outset what mature themes (as far as I am concerned) are not. All too often, particularly on TV (notably "Torchwood"), but also in RPGs books, mature themes seems to mean nothing more than boobs & blood. Oh, and the ability to say "fuck". (This is particularly amusing when applied to WotC's (3e) "Book of Vile Darkness" and "Book of Exalted Deeds", where they stated "mature themes", meant "boobs & blood", but then chickened out in the artwork to include not much of either. And indeed, when The Valar Project actually did try to publish "The Book of Erotic Fantasy" under the d20 license, they proceeded to change the license to try to block them!)
Anyway, I have absolutely nothing against boobs, blood, or expletives appearing in film, TV, books, or any other medium. But I would hardly consider "Piranha 3D" as particularly 'mature'!
So, then, what does "mature themes" mean? Well, I have a few thoughts...
Character Nuance: A lot of RPG characters are essentially one-dimensional. This is especially noticable in something like D&D, where motivation seems to be little more than "kill things and take their stuff" - which is pretty hollow, especially when you realise that most of that stuff is desirable purely because it enables you to kill things better. To that end, "Vampire: the Masquerade"'s concept that "a beast I am lest a beast I become" is quite interesting, since characters must constantly toe the line or lose themselves. "Black Crusade", in campaign play at least, has a similar conceit at work, which is quite fun. But even in one-shot play, characters each have a 'Pride', 'Disgrace', and 'Motivation', which should provide a little more depth.
Likewise, I think (I hope) that the NPCs are a bit less one-dimensional. There are no (well, few) clear "good guys" and "bad guys", various options for alliances are available, and even those characters who are insane do at least have some sort of motivation. (Note to self, though: must remember to bring that out in the game!)
The Gloves Are Off: This one may well not come up, but there will be absolutely no fudging of rolls to keep characters alive, and no fudging of rolls to keep characters playable. In many games, a permanent injury to a character could make that character no longer fun to play. Indeed, even just the loss of key equipment can cause some players to write off a character. There's a temptation, therefore, not to use creatures like Rust Monsters or energy drain attacks, or to fudge rolls to ignore critical results causing the loss of a limb.
Not here. Not this time.
A Fractious Party: Again, this depends on the characters that are selected for the campaign. However, some of the characters have back-stories that will make them less likely to seek certain solutions, others have competing goals. And, of course, the characters are all (nominally at least) followers of Chaos, and so inherently untrustworthy. So, where most of my one-shots assume a mostly united party with, at worst, mild disagreements, it's entirely possible that this lot will find themselves in outright war. And, frankly, that might be more fun than the prepared adventure anyway!
Shades of Gray: This one is actually probably most fun with a party of Paladins, who do want to stick to "the right thing" (but each of whom have character flaws, each of whom has a subtly different notion of 'right', and so on). Then, you just throw a moral dilemma at them, and watch them grapple with it. (It doesn't work with 'regular' RPG characters, because very few of them ever adopt rigid codes of behaviour voluntarily. Mostly, any such code consists of the morality of expedience - the moment it becomes convenient, Batman picks up that gun.)
With a party comprised of Chaos Space Marines and Heretics, things are rather different - the characters are already essentially Evil (or at least, on the side of the Daemons), so unlikely to fret too much about mindless carnage.
So this one will necessarily have to not be about the PCs. Instead, I'll have to have the NPCs grapple with questions of doing the right thing, of making the tough sacrifice. Oh, there is one area where the PCs have a potentially tough decision to make, but I'll not go into that... spoilers!
(But for the most part, this last one will be off the table. I have another "Mature Themes" game provisionally scheduled for next year, so I'll probably play with this particular theme a lot more then.)
And I think that's about it. I'll try to report back next week on how the game went...