Operation Sinterklaas (2007)

From publisher burb:

The players learn that the nefarious Thule Society have put in motion a plot to travel to the North Pole and abduct Santa Claus. The players must give chase to make sure that Christmas remains safe for another year.

This adventure is designed as a one shot mission using pre-generated characters found in the HEX books, though your players can make their own characters if they prefer. Not all of those characters would be appropriate for this adventure, but there are more than enough. This adventure can handle up to eight characters, for reasons that will become clear later in the adventure.

You could also use this adventure as the starting point of an ongoing campaign or as a transitional adventure in an existing campaign taking your characters inside the Hollow Earth for the first time. At the end of the adventure, you will be given the option of having your players returned safely to the surface or leaving them inside the Earth to explore its mysteries further.