The Curse of Frogs
The Shepherd's Tower
An archwizard named Khazan was one of those that worked to raise Castle Ravenloft (alongside Artimis the architect and teams of engineers and laborers). The wizard also constructed a private island on Lake Baratak, with an earth-and-gravel causeway inaccessible to any without the appropriate magics.
Currently, Rudolph van Richten has invited you to the tower, citing it as his base of operations and giving you the pass-phrase.