A heavy storm boils up while the players are on the road. They head for cover in a remote wayside shrine, from which signs of life can be seen. As they approach, a bolt of sudden lightning blasts down form the sky, striking the roof of the shrine with a crack and setting the place ablaze. A cry for help can be heard inside. The players rescue a young terrified cleric form the burning wreckage. He is injured, and the players take up the quest to get him to a doctor or some medical help.
Along the way to the nearest city/town, strange things start to happen to the party. The truth is that the young curious cleric had tried a certain spell, and failed, causing a dangerous series of events to unfold, allowing a shadowy demon to make its way into the plane. The cleric will keep this info hidden from the players as long as possible.
The shadowy demon will hunt down his summoner (and by association the players) until the problems is revealed and dealt with.
Maybe the young cleric was too curious and wanting to prove his magical might? Maybe he had been taken under his wing by another, more powerful sorcerer which the players must find to bring about an end to the shadow? Maybe the players leave the cleric in care, and later find out the cleric is found dead? does the shadow keep following the players? possibility of using the demon spectre across a few scenarios, always on their trail...
Will the players have to battle the spectre in an all out fight? Will they have to quest to find a spell/magic/item to help them banish the creature? Will the demon overcome the young cleric?