SplinterGlossary: Legendary Card
Legendary cards are most times impressive just by looking at them. But they are very powerful in battle too, if they are used correctly. In most cases, however, they won't win a battle all by themselves. It takes 11 copies to max out a regular legendary card (4 copies for a gold foil legendary, if one is lucky to have 4 GFL of a kind).
See also Card Rarities.