Mythical Beasts are built around Spell Counters, which is what gives them the ability to use their effects
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Activating the ability that turns it into a creature while it's already a creature will override any effects that set its power and/or toughness to a specific number
The inkwell is then used to summon other characters, items, and play action cards, creating a natural ramp up in resource costs as most players choose to ink something every turn
The most famous example is the Illustrator Pikachu, a card released in the 1990s that routinely goes on auction for millions of dollars thanks to its limited release
Parker and Spider-Gwen to complete the first movie's main cast (though there's plenty of time for those to potentially be revealed themselves before the set launches in September)