There is more than one way to make a Mystery card. I toyed around with a few iterations, and found a neat idea.
The rules for Mysteries are as follows:
1. You start the game with a Mystery deck. The first Mystery is turned face up as the game starts and you may solve the mystery on your turn if you meet the criteria. When you do, you get the reward, you exile the Mystery and reveal the next one.
2. You may solve more than one Mystery each turn, if you meet the criteria.
3. Once the Mystery deck is depleted, it is out of play for the rest of the game.