Dungeons. Heroes invade them and clear them out. That's been done. A mighty lich maintains an underground complex, mainly as a money-spinning operation. That's been done. You're the support staff. Well, that's been done too. (If you've not read http://www.dungeond.com/ yet, do.) We're gonna use a variant of this, too: http://www.youtube.com/watch?v=dsnvANYBRWo (YouTube?, "Play Dirty: The Dirty Dungeon" by LordStrange? (who is John Wick, of wickedthought, in case youtube hides it again)
The Setup: The players choose heroes. Then they start researching the dungeon (as per John Wick's video, above - anything they say is true - stated things add dice to the pool - detailed things add an extra die, dangerous things another extra die - every few minutes a glass bead is added, stop when it stop being fun. layer can spend dice as bonus at any time, GM can spend beads to add complications and cange known facts.) The heroes make their approach to the dungeon. And we stop there.
Within the dark heart of the dungeon, the big bad (who, if the players haven't changed it, is a liche) prepares for their coming. He creates, in time honoured fashion, dark duplicates of the PCs - identical in all respects except for having a card in their pocket which says "You are now: EVIL" and sets them off to defeat the originals.
The players now switch pretty freely between the good and the evil groups. The evil group sets a trap or ambush. The good group evades or defeats it. and repeat.
The basic dungeon, if the players don't come up with anything more interesting. (They will, but you can mine bits of this)
The room of chains. A large room filled with moving chains which connect various bits of machinery elsewhere in the complex. The chains move slowly and are under great tension. The good group could decide to destroy these, or gum up the works - but this is very dangerous. The evil group might do the same as a trap - they know more about how it works - but they'll get in trouble with the boss. Running along the chains are many intelligent rats, who take messages throughout the complex and talk in annoying squeaky voices.
The lava pit. Because every overlord should have one, this room has a tiny rickety bridge suspended over a chasm of lava, from which waves of heat roll. Actually, the dungeon is on a budget and the chasm is abut a foot deep and the illusion is maintained by a gnome kept in a small cage. There is a trap in the middle of the bridge, those who live in the complex know to just walk across the 'chasm'.
The large gong. A large gong is set upon a dias in this otherwise featureless room. The doors to the room are very sturdy, but open. This is the mess hall and pay room. Striking the gong causes all the monsters within hearing range to wander towards the room for their feed/pay. Both groups can surely come up with something interesting here.
The mystery box-room. A closet which functions much the same as a 'whatever'-of-many-things. Open the door, remove whatever random item appears within, close the door again. PCs closed within the cupboard will immediately swap place with their double. Sounds interesting. Do be sure to let us know how it runs. --CH