From the back of the book:
Return to Vézay
Rallying in the desecrated Church of St. Lazare, the forces of Hell have opened a new gambit for the souls of Vézay. The magi are invited to this quiet village to celebrate Easter week, and to solve the puzzle of an enchanted pouch of herbs. Witchcraft is suspected, but that soon becomes the least of the characters' problems as their immortal souls are placed in grave peril...
This official game story by Jonathan Tweet (co-author of the Ars Magica game) is written as a sequel to "The Ghoule of St. Lazare," a story which appeared in Tales of the Dark Ages (also from Atlas Games). While playing the adventures together may increase a troupe's enjoyment, Festival of the Damned stands well entirely on its own, as a masterful tale of mystery and temptation in the Middle Ages.