Gravelgill Duo 

Creature — Merfolk Rogue Warrior
$0.11 #30048 0.05 18.5 years
Whenever you cast a blue spell, this creature gets +1/+1 until end of turn.
Whenever you cast a black spell, this creature gains fear until end of turn. (It can't be blocked except by artifact creatures and/or black creatures.)
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God-Pharaoh's Faithful80%
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Gearsmith Guardian78%
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"This creature gets +2/+0 as long as you control a blue creature."
Hydro-Man, Fluid Felon78%
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"Whenever you cast a blue spell, if this card is a creature, he gets +1/+1 until end of turn."
Briarberry Cohort78%
$0.18↑#262870.0518.5 years
"This creature gets +1/+1 as long as you control another blue creature."
Wingrattle Scarecrow78%
$0.52↑#11423↑0.0018.5 years
"This creature has persist as long as you control a black creature."
+1 more matching line
Emberstrike Duo76%
$0.09↓#303640.0018.5 years
"Whenever you cast a black spell, this creature gets +1/+1 until end of turn."
Quirion Dryad76%
$0.09↓#252940.1325.7 years↑
"Whenever you cast a spell that's white, blue, black, or red, put a +1/+1 counter on this creature."
Thistledown Duo74%
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"Whenever you cast a blue spell, this creature gains flying until end of turn."
Minotaur Tactician74%
$0.06↓#321310.0025.4 years↑
"This creature gets +1/+1 as long as you control a blue creature."
Merrow Bonegnawer73%
$0.18↑#17105↑0.0018.2 years
"Whenever you cast a black spell, you may untap this creature."
Glade Gnarr72%
$0.06↓#319180.0025.4 years↑
"Whenever a player casts a blue spell, this creature gets +2/+2 until end of turn."
Merrow Levitator72%
$0.22↑#257280.21↑18.2 years
"Whenever you cast a blue spell, you may untap this creature."
Ashenmoor Cohort72%
$0.18↑#316840.0018.5 years
"This creature gets +1/+1 as long as you control another black creature."
Questing Druid // Seek the Beast71%
$0.35↑#8702↑0.113.1 years↓
"Whenever you cast a spell that's white, blue, black, or red, put a +1/+1 counter on this creature."
Scornful Aether-Lich71%
$0.21↑#317380.0517.7 years
"
: This creature gains fear and vigilance until end of turn."
Ana Sanctuary70%
$0.21↑#18585↑0.21↑25.4 years↑
"At the beginning of your upkeep, if you control a blue or black permanent, target creature gets +1/+1 until end of turn. If you control a blue permanent and a black permanent, that creature gets +5/+5 until end of turn instead."