Since demons are from other dimensions, they might also be utterly non-understandable. They might (and probably will) function completely out of every established concept of logical, emotional and intellectual thought processes. You could give the PC's a nightmare ride, screwing with their minds and making them paranoid, not trusting anyone, and let them dig a deeper and deeper grave as they try to solve everything the ordinary way.
Thats what I thought of anyway...
/ninja
You mean like the Vlathachna...?
Yes, they've done that too! (Admittedly, it was ten years ago in one of their earlier adventures - that particular game stands out as being my first two-parter (for my eighteenth birthday) where they teamed up with their other characters form another party we played. So perhaps it's time I had another go...)
For the record, their past adventures read something like this:
The first adventure brought them into contact with the Kra'Vac (alien race from the Full Thrust wargame background), employed by a human who would become a recurring nemesis.
The second one I ran (someone ran a quick one-shot inbetween) they ran into Zhainmoth, leading into the third quest which was the aformentioned cross-over/Vlathachna/Zhainmoth game.
Their forth had them fleeing their nemesis, now revealed as an Imperial agent and Imperial forces, running to pirates, stealing their ship and crash landing on the capital world of the Army Of The Red Spear (Undead).
After escaping that, but being captured, their fifth adventure had them unwilling helping the Aotrs against the super-high tech technological might of the Blasted Legion - or Blastaron - Empire.
Somewhere before this point the Jedi did a quick solo adventure, rescuing his fiance from the cluthces of the Empire and the party's nemesis.
They then ended up fighting off aliens immune to energy and projectile attacks in a subterran cavern complex to rescue the Jedi's fiance (
again - she joined the party after this - it was safer!) and dealing with two Brotherhood Monks in adventure six.
One of the players ran the next game, where they faced off against the Acrodinians (aliens) and Liam was kidnapped.
There followed a short side adventure, rescuing him from the clutches of the New Eire, who wanted to turn him into a potato clone - a mindless slave whos only function was to grow potatoes - for high crimes treason against the potato, where they once again, breifly encounter Imperial, Brotherhood and Blastaron forces. (I told you you'd be better off not knowing!)
(I think they may have been one or two more adventures other folks ran between them and my next one.)
The seventh quest I ran had them chasing purple-people eaters and avoided the Imperials (tracking them after last game) and the Brotherhood and their monestary of Monks specifically created to the kill the party. (Gillman was turned in at the start of this game.)
I think there was another game between that one and my next one, which had them chased down by the Blastarons and where they found a Sienetic Harbinger base. A bit of exploration later, they found something (a teleplat chamber) that might lead them to an even larger facility...
Meeting up with the enemies of the Blastarons who might be able to work out where the new facility was, they endured a lengthy set-piece chase scene (played mostly for laughs, but with just enough seriousness that at least one of the player really thought I was going to wipe them out) where they were chased by the Entire Blastaron High Command, the Empire, the Brotherhood and
Megatron. Yes, Megatron! They then ended up exploring another small Harbinger base, leading clues towards the mystery base they encountered before.
Their next adventure pitted them against Klingons in a bid to avenge their Klingon ninja's father's murder -and they ended up unwittingly helping the Aotrs once again.
Then another Harbinger base, this time the culmination of the Harbinger arc.
After causing this much rucus, the party legged it to the fringes of the galaxy, ran into Liam and Gillman again, the pair were pirates attacking an unknown alien frieghter. The PCs followed the frieghter and found a strange alien facility (which in the end Liam ended up snapping and starting a fire-fight in.) The follow-up adventure found another of the alien's facilities, but revealed nothing more about their motives or what sort of being was pulling the strings behind the technoligcal/magical/psionic constructs they encountered.
And finally they went in search of lightsabre crystals (thanks to a delightfully lousy Weapon Tech roll, the Jedi now has a
tick-sabre, which sounds like and has the colour of urine...) and ran into Orcs.
To sumamrise then, they've fought Kra'Vac, Imperials, Undead, Blastarons, Acrodinians, Klingons, androids, Harbinger robots, alien constructs, Orcs, pirates, Brotherhood Monks (including one Sith), Zhainmoth, Zhainmoth Cultists, amorphous clouds of purple people-eating gas, invulnerable-to-all-but-melee aliens, people with an unhealthy potato obession, a few law-enforcement types, numerous alien animals and at least one Vlathachna and one Decepticon...
So you can see they've had a fairly broad history of encounters and enemies. (I wasn't kidding about the "Simpsons did it" much, was I!)
Still, all this has made me think it's about time I dusted off a few more of those old adversaries for next time!