- Foxglove Manor was built by a wizard long ago, but is now inhabited by a new noble family. The evil influence of the long-dead wizard has corrupted the family. (Pathfinder).
- The daughter has been lured to an illusory palace underground by the Silver Eyed Prince (Hellboy, "Christmas Underground).
- The younger son, a tortured artist, has been building a statue of an archdevil, assisted by an imp (Hellboy, "The Temple of Moloch").
- The oldest son has been driven to murderous rages by the influence of the old wizard's writings (Pathfinder, Aldern Foxglove's character).
- The father lies on his deathbed, hoping he can help his daughter find peace by passing on the family holy symbol ("Christmas Undergound")
- A dead tree with fresh black leaves is in the courtyard, apparently a source of evil (Andy Collins' Bloodlines campaign blog).
I was worried that I would feel compelled to make the adventure stick to the stories I was using as inspiration, but that got demolished real quick. The players did things completely different than how I expected. I had peppered foreshadowing into certain areas, but more often than not the PCs fought a monster, then found the "clue" I had left. They entered the hedge maze before the castle (I wasn't expecting a minotaur in the party would be irresistibly drawn to it...).
Since I'm going for a sandbox feel, I've embraced this rather than fought it. For example, in the Hellboy story "The Temple of Moloch", Hellboy discovers that a tortured young artist has been sculpting a demonic figure. The man goes into his workroom at night and locks the door. So Hellboy waits in the room until nightfall and witnesses a small impish creature crawl out of the ground and direct the artist's hands on the statue. When Hellboy intervenes the statue comes alive, they fight and Hellboy wins. In my adapted version in the castle, the players killed the artist as soon as they met him! They thought he was a vampire, and I can't really blame them. They later investigated the statue and decided to destroy it, just before nightfall. So instead of having a fight against a possessed statue, I decided that the imp called the artist's soul to punish the PCs, and they fought a wraith instead.
I never had a true "Plan B", but I went in with the assumption that my "Plan A" was disposable if the players did something unexpected.