The PC's find a map to an abandoned underground lair - including some description of the resident of the lair. They recognise the resident and know that he is long dead (perhaps a high level MU). The map details the caverns to a great extent (perhaps leaving out some key rooms). The map also neglects to mention the many traps set throughout the lair - or perhaps mentions one or two giving the PC's a false sense of security. As for monsters, are undead boring? They still be around from the MU's days. Perhaps rodents, snakes, spiders as well. For the main villain a wight or perhaps a very minor demon (trapped on this plane with the appropriate wards - "You have entered a room with a large circle engraved on the floor. You notice this circle because it lights up as the fighter walks into it. The room suddenly becomes very dark and you hear a low growling laughter."). Or perhaps the MU is not dead, only very old and quite insane - thus maybe he has neglected to memorise his higher level spells and sits quietly on his throne waiting to die (thus not such a formidable opponent). Or perhaps his last opponent turned him into an ogre or a doppelganger or whatever you want (maybe using divine intervention to end his spell casting ability).