It was the curse.
The curse of a defrocked friar that set a group of murder mystery readers re-enacting a new mystery at an abbey, now owned by a wealthy industrialist, on an adventure they would remember the rest of their lives.
The curse on the abbot doomed him to walk the halls of the abbey every solstice and murder someone in the abbey.
When one of the group dies from a heart attack and the son of a wealthy industrialist is found murdered, they suddenly begin to worry would be next.