Neera, Wild Mage

A gamble engine that only looks like a value engine, and the tension between those two identities is what you actually build around. The offer is deliberately lopsided: bottom the spell you meant to cast, then dig through your deck until a nonland surfaces and cast that one for free instead. You do not get the card you chose; you get whatever the library hands you, which means your deck, not the spell in your hand, is the real input to the engine. The 2/7 body does quiet load-bearing work, wide enough to survive most attempts to kill her before you ever pull the lever. And the "only once each turn" clause is the governor on the whole apparatus: one flip per turn, so every activation has to earn the swap of a known quantity for a random one. That single restriction reframes deckbuilding around variance in a way most cast-triggers never do, because your payoff depends on how consistently top-heavy your average hit is, not on how many triggers you can stack. Load the deck with expensive nonland bombs and she becomes a cost-cheat that flatly refuses to let you aim; load it with cheap spells and she becomes a coin flip you keep taking because the worst case is capped at "I cast something other than what I paid for." The card commits fully to that trade-off and asks you to make peace with never being certain.