The characters must either calm the populace so that the portal can dissipate, or take on Nightmare themselves. Since the overlords of his own realms are to powerful for him to take on he has decided to expand outside his own dimension - and into ours. He passes a group of his greatest warriors through first to test the waters. If these supervillains are not defeated by the characters - they will return to their lord and he will begin the invasion. Ground troops and war beasts will come through temporary portals and make things very difficult for the players.
Police suspect a serial killer, and the media are hyping it up.
Every murder has blood everywhere, but no valuables have been taken and there are no connections between the victims. A few witnesses have reported seeing a youth or midget fleeing the scenes wearing a red stocking cap, black pants, and large boots. There is an old woman named Agnes Scrimm who lives in the area of the murders and claims that the murderer is not human, but an ancient Celtic faerie. Police and the media have largely ignored her. The heros decide to hear her out.