Dungeons And Dragons RPG Game Master's Toolbox: Luck Deck - Rules of Play says:
This deck of simple cards adds a whole new dimension to your role playing game, giving the players a new Luck variable that is very simple to keep track of and extremely fun to utilize — if luck plays out in your favor, that is. Simply put, if you roll a 20, you’ll have good fortune to cash in later. If you roll a 1… well — try not to roll a 1
26 good luck cards for use by the players
26 bad luck cards for use by the GM
Roll well and you’ll have an additional arsenal against your foes, roll poorly and bad fortune awaits you!
Dungeons And Dragons RPG Game Master's Toolbox: Luck Deck Video:
Don't just take our word for it! Check out this video showing the Critical Hit Deck!