The condition on a legendary instant or sorcery requires you to control a legendary creature or planeswalker in order to cast such a card
Playing Christopher Robin, Adventurer alongside a large collection of other character cards is a sure-fire path to winning.
Suppose you play against a deck like Daretti, Scrap Savant that can assemble combos like: cheat Darksteel Forge and Mycosynth Lattice into play, and follow that up with mass artifact destruction
You can buy gems in Arenas store for actual cash (or cash alternatives), earn them through Draft rewards, or occasionally through events
Other cards in this deck, like Tentacruel and Golduck also bring consistent damage in the active spot, making this deck have a focus on offensive play
The illustration does not shy away from the disturbing implications of a sentient being held captive and weaponized by a criminal organization, and that willingness to engage with the more mature and morally ambiguous aspects of Mewtwo's character gives the card an emotional weight and narrative gravity that most Pokemon cards do not attempt