Diffusion Sliver 
Creature — Sliver
$2.81 #4264 0.33 12.1 years
Whenever a Sliver creature you control becomes the target of a spell or ability an opponent controls, counter that spell or ability unless its controller pays 
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Cards similar to Diffusion Sliver expected a card that isn't here?
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Unsettled Mariner80%
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"Whenever you or a permanent you control becomes the target of a spell or ability an opponent controls, counter that spell or ability unless its controller pays ."
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Parnesse, the Subtle Brush79%
$0.40↓#16680↓0.244.4 years↓
"Whenever you or a permanent you control becomes the target of a spell or ability an opponent controls, counter that spell or ability unless that player pays 4 life."
Amulet of Safekeeping78%
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"Whenever you become the target of a spell or ability an opponent controls, counter that spell or ability unless its controller pays ."
."
Frost Titan76%
$0.46↓#9673↓0.20↓16.1 years↑
"Whenever this creature becomes the target of a spell or ability an opponent controls, counter that spell or ability unless its controller pays ."
."
Reality Smasher74%
$0.71↓#10594↓0.3310.6 years↓
"Whenever this creature becomes the target of a spell an opponent controls, counter that spell unless its controller discards a card."
Opaline Sliver72%
$2.05↓#10654↓0.50↑19.9 years↑
"All Slivers have "Whenever this permanent becomes the target of a spell an opponent controls, you may draw a card.""
Reidane, God of the Worthy // Valkmira, Protector's Shield71%
$0.44↓#6365↓0.71↑5.6 years↓
"Whenever you or another permanent you control becomes the target of a spell or ability an opponent controls, counter that spell or ability unless its controller pays ."
."
Leeching Sliver70%
$0.72↓#10974↓0.22↓12.1 years
"Whenever a Sliver you control attacks, defending player loses 1 life."