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If your group is reasonably good or anything(like, good clerics and paladins, etc) then make them make a tough role playing decision...Let's say that all of a town's children disappreared mysteriously and get them to go to some castle or dungeon. Then, have them find the children, but unleash a series of traps that either kill children or kill PCs. Hopefully, you should have them get to some kind of mastermind that orchestrated this, so that a lot of the kids survive with the npc's to see the evil guy. He re-captures them and floats them over a pit or vat of acid or something that is lethal 100% to the kids. Then, have him say something to the effect of "Let me go and the kids live, move closer and they die.." but dont' give them any way to do both. If any of them move, he kills the kids and warps out, doors lock, and a ton of minions swarm the players. Even if they do nothing, he drops them and warps out, and ton of minions swarm the players biggrin . If their goal was to save kids, they fail and now have a new archenemy biggrin

that's creul. I like it! twisted

Another idea I had once was that the PC's on a rescue mission to rescue a heap of children are trapped in this maze by the villain, and the villain says to them;

"For every dead end you hit, one child dies." [The villain can be on a high up platform, or speaking from elsewhere with Message, or whatever.]

Make sure the villain is protected against retaliation by the PC's if he/she is there. [Protection From Arrows, Stoneskin, Greater Spell Immunity, Spell Turning, Globe of Invulnerability etc.]

And every time they hit a dead end, a horrible childs scream is heard.
[For maximum effect, make the scream yourself suddenly or play it from a hidden CD player really loud]. Make the scream again of they take more than ten seconds to choose a direction. The maze must be fairly complicated, and Gaming Groups who are too lazy to map are punished by this more than others. If you are TRULY sadistic, then using moving walls, pit traps and roving monsters would enhance this.

And then when they get to the end, a cacophany of screams are heard, as the villain kills all the remaining children. Then, from the ceiling drops the mangled corpses of those they were sent to rescue. As they hit the ground, the corpses hit a place on the ground that triggers an Animate Dead Spell. [Caster level equal to half the amount of corpses]. The mutilated, grotesque, undead remains of the children they were sent to rescue are now shambling towards them with intent to kill.

HAHAHAHHAHAHAHAHHAHAHAHAHAHAHHAHAHAHAHAHAHAHHAHA...HA....ha.

[Of course, this would work with Good PC's more than Evil ones.]

I'm a Chaotic Evil Guy in a Lawful Good Guy's body.

sic the tarrasque on em'. xd tarrasque solves everything!

If you really want to kill the party take any monster from the epic level handbook and go nuts. Guaranteed s**t kicking of the party.

Players versus 4 dire tigers advanced to 20 hit dice.

Scare them? You could say mordenkainen's disjunction XD

I've never had to do much to scare the crap out of my party, they hear a story/rumor about mindflayers and they're kilometers away from the town in no time...yet they insist on fighting dragons...? They really confuse me sometimes.