Ferocity 

Enchantment — Aura
Enchant creature
Whenever enchanted creature blocks or becomes blocked, you may put a +1/+1 counter on it.
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Cards that do something similar expected a card that isn't here?
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Infinite Authority 87%
$11.91#30063
"Whenever enchanted creature blocks or becomes blocked by a creature with toughness 3 or less, destroy the other creature at end of combat. At the beginning of the next end step, if that creature was destroyed this way, put a +1/+1 counter on the first creature."
Gift of the Woods 84%
$0.17#26477
"Whenever enchanted creature blocks or becomes blocked, it gets +0/+3 until end of turn and you gain 1 life."
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