The mind of a wizard is more open to the more esoteric elements of his surroundings - this is why he is able to perceive the weave of the Great Tapestry that is the multiverse, and pull at the strings. This increased sensitivity can manifest in several ways.
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!