What is the Legendary Charge gauge in the Card Shop?

For every pack you opened, you’ll get a Legendary Charge point. Keep on accumulating this and on the 30th pack you opened, it will trigger a Legendary Charge where you’ll get a random Legendary Card at 100 chance%. After that, the Legendary Charge gauge will reset. You can still repeat this process as often as you want (and afford).

Legendary cards are chosen from a pool that excludes cards you already own. Therefore, the rarity % differs depending to each player's collection status. If you already possess all Legendary Cards in the list of "Possible Cards" in a banner, any Legendary card you receive will be randomly selected from the pool of all Legendary cards.

At the moment, only Core Cards are available for the initial release of the game. You can see a list of some of these cards from the game’s official site.