You get a 12/12 trampling monster, plus a powerful and flexible ability that triggers whenever you attack, so you dont have to wait until Ghalta and Mavren itself attacks to get the benefit
They can be purchased in bulk online or at local hobby shops
How does this work here as the leader
Once you have these cards, you're also free to customize your deck as you see fit though you need to have a minimum of 40 and maximum of 60 cards per deck, not accounting for any extra cards like Fusion or Xyz monsters
All being well, all youll need to do is pop one more Energy onto Gyarados ex, and you can begin dishing out Rampaging Whirlpool
Collectors usually send cards featuring popular Pokmon like Charizard and Pikachu to get graded, as those fetch a higher price