The party is walking through the forest at night when they come upon a clearing. Half a dozen black-robed corpses lie scattered across the ground, and a pentagram of blood is shoddily drawn in the dirt.
It seems these people summoned something they could not control. Whether or not the demon returns is up to the GM, but it would be just as rich if the demon never returned, and for the rest of the night, every stray sound or odd shadow will be jumped at!
A dark necromancer has discovered a way of making the magical energy cost of resurrection less than the energy he can gain by draining their life force. Given a sufficient supply of mind-drained minions, ultimate power beckons.