Aberrant Manawurm 

Creature — Wurm
Trample
Whenever you cast an instant or sorcery spell, this creature gets +X/+0 until end of turn, where X is the amount of mana spent to cast that spell.
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Cards that do something similar expected a card that isn't here?
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Renegade Bull 99%
$0.23#15025
"Whenever you cast an instant or sorcery spell, this creature gets +X/+0 until end of turn, where X is that spell's mana value."
Erratic Cyclops 99%
$0.20#17911
"Whenever you cast an instant or sorcery spell, this creature gets +X/+0 until end of turn, where X is that spell's mana value."
Fire Urchin 97%
$0.08#24988
"Whenever you cast an instant or sorcery spell, this creature gets +1/+0 until end of turn."
Frolicking Familiar // Blow Off Steam 97%
$0.24#7735
"Whenever you cast an instant or sorcery spell, this creature gets +1/+1 until end of turn."
Aven Wind Mage 97%
$0.16#22687
"Whenever you cast an instant or sorcery spell, this creature gets +1/+1 until end of turn."
Spellrune Painter // Spellrune Howler 97%
$0.17#20131
"Whenever you cast an instant or sorcery spell, this creature gets +1/+1 until end of turn."
Expressive Firedancer 97%
$0.08#22911
"Whenever you cast an instant or sorcery spell, this creature gets +1/+1 until end of turn. If five or more mana was spent to cast that spell, this creature also gains double strike until end of turn."
Charmbreaker Devils 96%
$0.34#7255
"Whenever you cast an instant or sorcery spell, this creature gets +4/+0 until end of turn."
Elemental Mascot 96%
$0.10#21024
"Whenever you cast an instant or sorcery spell, this creature gets +1/+0 until end of turn. If five or more mana was spent to cast that spell, exile the top card of your library. You may play that card until the end of your next turn."
Colorstorm Stallion 95%
$0.43#14924
"Whenever you cast an instant or sorcery spell, this creature gets +1/+1 until end of turn. If five or more mana was spent to cast that spell, create a token that's a copy of this creature."
Kiln Fiend 94%
$0.21#16641
"Whenever you cast an instant or sorcery spell, this creature gets +3/+0 until end of turn."
Mercurial Geists 94%
$0.04#25314
"Whenever you cast an instant or sorcery spell, this creature gets +3/+0 until end of turn."
Aquatic Alchemist // Bubble Up 92%
$0.20#14399
"Whenever you cast your first instant or sorcery spell each turn, this creature gets +2/+0 until end of turn."
Deluge Virtuoso 91%
$0.15#22270
"Whenever you cast an instant or sorcery spell, this creature gets +1/+1 until end of turn. If five or more mana was spent to cast that spell, this creature gets +2/+2 until end of turn instead."
Festival Crasher 91%
$0.27#19713
"Whenever you cast an instant or sorcery spell, this creature gets +2/+0 until end of turn."
Ghitu Amplifier 91%
$0.04#24902
"Whenever you cast an instant or sorcery spell, this creature gets +2/+0 until end of turn."
Storm Skreelix 91%
$0.16#17662
"Whenever you cast an instant or sorcery spell, this creature gets +2/+0 until end of turn."
Wee Dragonauts 91%
$0.14#17817
"Whenever you cast an instant or sorcery spell, this creature gets +2/+0 until end of turn."
Eager First-Year 90%
$0.04#27236
"Whenever you cast or copy an instant or sorcery spell, this creature gets +1/+0 until end of turn."