From Bulbapedia, the community-driven Pokémon encyclopedia.
The Legendary Cards are the extremely rare and powerful cards from Pokémon Trading Card Game and Pokémon Card GB2: Here Comes Team GR! that were mostly guarded by the Grand Masters.
If the player wants to earn the extremely rare and powerful cards, he/she, in a similar style to the Pokémon League, must defeat the eight Club Masters to obtain their eight respective Club Medals. After this, he/she is able to enter the Pokémon Dome, where the Four Grand Masters must be defeated. It is possible to defeat them multiple times, to obtain multiple copies of the cards.
Only one of the Legendary cards had been released into an actual card - in the packaging of the Pokémon Trading Card Game, which is for Japanese version only. This card is the Legendary Dragonite. The other three Legendary cards in the game cannot be reproduced faithfully for the TCG, as their card effects involve the CPU choosing a Pokémon at random to be affected.