For example, if you discover 4 and reveal Galvanic Giant (an adventurer card from Wilds of Eldraine with a mana value of 4), you could cast Galvanic Giant, but not Storm Reading (its Adventure, which has a mana value of 7)
Will-o'-the-Wisp is a good example of this, as is the original Mana Vault
It makes things happen in a format prone to board stalls and encourages interesting discussion at the table
Over three turns this will essentially have the same effect as even a Commune with the Gods, which is arguably the most powerful of your self-mill spells
These cards are not part of the main set, instead only legal in places like Commander or other formats where they were already playable
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