Shadows over Nentir Vale is the first set of Dungeons & Dragons Fortune Cards. The cards are intended to provide players with additional options on their turn, and they're basically new powers or power modifiers.

From the publisher's website:

Encounter Fortune.

Dungeons & Dragons Fortune Cards, sold in booster packs of 8 cards, give players fun new ways to survive the challenges of the D&D Encounters in-store play program, as well as their home campaigns. These cards give characters fun, temporary benefits that feel different from the benefits gained from powers and feats, without adding undue complexity to the D&D game.