The shadow card was the second card that Sakura caught. It was the very first time she used the key to get her wand...as she said. This was the first episode with Madison in it as her sidekick and best friend.

What happened was that Shadow stole everyone's shadow and when Sakura tried to get the card, it released the shadows against her. Shadow's power is to restrain cards as well as attack them with shadows and shadow powers.