These cards were made by Steve Jackson, of Fighting Fantasy fame, and include many of the artists who illustrated that series. They included a Magic-like card game as well as some puzzles ('quests') to solve. Good all around fun!

The backs of the treasure cards, when decoded, read:

There are actually 140 cards in the set. The extra card is: The Emperor of Vangoria. Get you free copy by translating these messages.

Then it proceeds to tell you to mail in your 8 treasures cards etc. like with the other quests.