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!
The babies of a small village has been disappearing lately, rumour has it that a demon living in the forest has taken them. The monster has the appearance of a shriveled old woman. A hag, disheveled, with maniacal appearance, wild-looking hair, and an oversized gaping mouth. Long pendelous breasts. The villagers say she eats the newborns and has sharp claws that are created for mangling. No one dears to challenge her, enter the PCs.