Demon invocation in the Wasteland

The city in the wastelands where you all are resting is peaceful at night. Suddenly an earthquake wakes you all up when the moon is in its higher position. In a close hill, a red shine, like the one fire produces, appears, preceding an army of demons that fly torwards the city. The captain of the guard tells you that some kind of cult threatened the city some time ago. They will protect the people, but you all must find what is producing that glow and prevent more demons from attacking the city. In these ritual place, undead, demons and the members of the cult awaits you for the final battle.