All-Seeing Arbiter 
Creature — Avatar
$0.34 #11584 0.19 4.5 years
Flying
Whenever this creature enters or attacks, draw two cards, then discard a card.
Whenever you discard a card, target creature an opponent controls gets -X/-0 until your next turn, where X is the number of different mana values among cards in your graveyard.
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Cards similar to All-Seeing Arbiter expected a card that isn't here?
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+1 more matching line
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"Whenever this card enters or attacks, draw a card, then discard a card."
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"Whenever this creature enters or attacks, it connives."
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"Whenever this creature enters or attacks, you may discard a card. If you do, draw a card."
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$1.89↑#1582↑0.31↑2.9 years↓
"Flying"
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$0.14↓#19624↓0.00↓6.3 years↑
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$0.22↓#109350.06↓2.3 years↓
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"Whenever this Vehicle attacks, draw a card, then discard a card."
+1 more matching line
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$0.33↓#19883↓0.07↓3.1 years↓
"Flying"
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$0.04↓#26972↓0.06↓2.7 years↓
"Whenever this creature attacks, draw a card, then discard a card."
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"Whenever this creature enters or attacks, you may discard a card. If you do, draw a card."
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$0.25↓#104730.00↓1.7 years↓
"Whenever you attack, draw a card, then discard a card."