The best thing I thought about was that from the point of view of the people who were in the group last year, they really DON'T know who the new players are, nor do they know what happened to the PCs of the people that left - good role playing opportunity. Go to Comment
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!