It is in Silver-Bordered World
Prompto Argentum Promptly engaging with haste, Promptos selfie shot ability helps you cast those more expensive spells which will make him important in limited, and maybe even some use in casual or commander decks
Unintended interactions have a tendency to generate emergent gameplay
Ezuri, Stalker of Spheres Ezuri, Stalker of Spheress initial proliferate trigger does seem a little expensive at three mana on top of the four cost
In gameplay terms, Vault 112 seems like a pretty decent energy producer that slows your opponents down, then becomes a powerful effect to spend your energy on, providing a free card which you can dig deeper for if you plug in some volts
To determine the total cost of a spell, start with the mana cost or alternative cost you're paying (such as a flashback cost), add any cost increases (such as kicker costs), then apply any cost reductions (such as that of The Earth Crystal's first ability)