The Pay What You Want Adventure - Gift of the Gods From Griffon Publishing Studio For Your Old School Horror Campaign

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This is a fantastic pulp style horror adventure for use with any old school horror style game set before the advent of  WWII.  Think Call of Cthulhu or Raiders of the Lost Ark here. Pulp generally has some style of horror lurking in the background.
It could be with little work be done as an OD&D style game.
This is an adventure with choices and the party is in control of the situation but barely.
The set up according to Drivethrurpg : 

This free adventure outline by Peter Schweighofer introduces readers to the exciting world of Heroes of Rura-Tonga, a roleplaying game supplement presenting source material outlining elements for a 1930s pulp campaign in the South Pacific and five, full-length scenarios. When the heroes travel to Antan Island in their seaplane to investigate rumors of a strange cargo cult, they stumble upon an artifact from an alternate future history. But as they're trying to unravel this paradox, they find themselves in the middle of a Japanese hunt for fleeing British spies. Can the heroes evade the Imperial Japanese Navy, rescue the British agents and their important intelligence, and unravel the mystery of the cargo cult's enigmatic artifact? Check out this free scenario to get a feel for the kinds of adventures you can play in the Heroes of Rura-Tonga game supplement and adventure collection available from Griffon Publishing Studio.

Gamers can easily adapt this Any-System Key adventure to their favorite roleplaying game rules. The Any-System Key describes character skills and task difficulties in terms that easily translate into nearly any game system. These terms do not link to any one game engine, but serve as general guidelines to give readers an overall sense how they fit within their own game's stat and difficulty scale. To use this scenario with your favorite roleplaying game system you'll need the copy of the Any-System Key included with the free adventure outline file; or you can download it free from Griffon Publishing Studio.

Using this Adventure 

There is a lot of room to play with this one. From edge of  Cthulhu style horror all the way up to using this as a means to getting your PC's to Carcosa this is one of those adventures that can be tuned to the horror situation that the DM loves and needs to use. I've had quite a bit of fun with this one and have used it as a prelude to an Actung Cthulhu game.
 This is a very well written adventure and easily adaptable to a number of situations and eras. There is quite a bit of room to play here if the DM is willing to invest a bit of time and energy into this one.