From the Module description:
Thirty Years ago Victor, second son of the Duke of Bannerwalls, imprisons his older twin Henri and takes the Throne of Bannerwalls for himself. Twice since then, Henri has escaped and (being quite mad by now) murders the first young girl he sets eyes on. Victor is quite amused by this, and always catches him and imprisons him again. This year though, the Duke has guests (the PC's) during the Winter Solstice Festival and Henri's untimely escape has stirred up a hornet's nest.
The locals are terrified, the Duke is unpopular with his subjects, and the situation is extremely tense. If the PC's do not do something, (anything!) for their Liege Lord, tragedy will be inevitable. The Duke is not interested in the PC's finding out the truth or any of his business. He has had them search frantically for the truth while he devotes himself to his own research into the Blue Moon conjunction, and distances himself from the proceedings, fully intent on blaming them and executing any of them as soon as he has the time.
For the PC's the disgrace, more than anything else, will be disastrous. What will you do...?
AD&D 1st Editions for FOUR Characters of 9TH Levels