Mizzix of the Izmagnus

Legendary Creature — Goblin Wizard    (CMC:4)

Whenever you cast an instant or sorcery spell with converted mana cost greater than the number of experience counters you have, you get an experience counter. Instant and sorcery spells you cast cost less to cast for each experience counter you have.

116 PZ1 • ENCliff Childs

™ & © 1993— Wizards of the Coast

Notes: TODO: Check rarity

Legal in: Legacy,Vintage,Freeform,Prismatic,Tribal Wars Legacy,Singleton 100,Commander

Oracle Text (click to copy):

11/4/2015
You finish casting a spell, including paying its costs, before Mizzix’s first ability triggers. The ability that gives you the experience counter will resolve before the spell that caused it to trigger.
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Mizzix’s last ability doesn’t change the mana cost or converted mana cost of any spell. It changes only the total cost you pay.
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Mizzix’s last ability can’t reduce the amount of colored mana you pay for a spell. It reduces only the generic component of that cost.
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If there are additional costs to cast a spell, or if the cost to cast a spell is increased by an effect (such as the one created by Thalia, Guardian of Thraben’s ability), apply those increases before applying cost reductions.
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The cost reduction can apply to alternative costs such as flashback costs.
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If an instant or sorcery spell you cast has  in its mana cost, you choose the value of X before calculating the spell’s total cost. For example, if that spell’s mana cost is  and you have one experience counter, you could choose 5 as the value of X and pay  to cast the spell.
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Experience counters are the second kind of counters a player can have, joining poison.
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All experience counters are identical, no matter how you got them. For example, the last ability will count experience counters that you got from the first ability, from another ability, from proliferating, and so on.
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Each game pack includes a card labeled “Experience” with the suggestion “Place your experience counters here.” This card isn’t required for play. It’s simply a convenient spot to put your experience counters, which can be represented with dice, glass beads, or other small items.