The Lich vs. Evil

The Plot

The lich is a good wizard who was forced to become a lich in order to remain around to counteract some powerful evil force. He/it spent the last years of his life directly restraining some powerful evil demon (make it something not quite physical, for example a demon of madness that manifests by making victims psychotically insane...evil human sacrifice cults start springing up all over the place and random people on the road start attacking out of the blue with no provocation, sort of like...gasp! PCs!)

So the Lich is at the bottom of some dungeon complex using spells and powers that are so far beyond the party's understanding that they can't perceive them, to hold the evil imprisoned. He/it is also keeping random strangers from wandering in and interfering. After so long a time, the lich just sort of drifted into undeath without really noticing (keeping a set of spells up constantly for years will do that to ya). The PCs manage to get the drop on the Lich when he's weakened and...

1. the evil gets loose.
2. the good lich's wizardly spirit manifests before it moves on to another, higher plane, and commends them for their actions in releasing him from his unwitting servitude to to undeath. He also says, "Well, I'm off to my retirement in elysium, the job's all yours, boys!"
3. If you're feeling charitable, give the players an inkling of what's going to happen, or some magic to help them to combat the madness demon (personal protection against the madness would be nice, although you could have lots of fun with blackouts and sleepwalking and the like if the PCs were as susceptible as anyone else). If you're not feeling charitable, have them find out the HARD way what the ol' spook's mysterious comments were in reference to. Maybe stick a scroll (that must be laboriously deciphered) in with the treasure, describing the madness demon and perhaps some ways that it can be fought.